Newbie question...

When you ride, do you intentionally try to keep your knees in close to the bike? Is there a benefit in doing so? Or, do you just let your knees flail about however they choose?

Knees straight. No wasted lateral motion or stress that way, efficiency of movement is critical. One of the sure signs of an inexperienced rider is when as I'm approaching someone from behind, I see their knees splay outward at the tops of the pedal stroke. I know instantly that at the very least they won't give chase as I pass them.

If your knees flail about, there are two possible causes. One is your saddle isn't high enough. The other is your have bio mechanical problems with your foot/lower leg. Often simple inserts can take care of that.

Your knees should be neutral unless you're compensating for an injury or have odd biomechanics.

Go to a fitter.

In a perfect world, your knees should go straight up and down over your feet. That said, My knees tap my top-tube, because I have very messed up ankles and in-step. My fit is not one that most people should copy. I've got angled shims under my cleats, both slanting the same direction, as well as a vertical shim under my right foot.

Tl;dr: Knees straight up and down.

Think of your knees as a pair of pistons. Just straight and down. Neutral, as Jandro stated.

hardcore.

Back in the '70s I read some books on racing and there was mention numerous times about knees touching or almost touching the top tube to provide better aerodynamics during TTs. Take note during the Tour and you will still see some riders doing this. I ride with knees in and slightly pigeon toed. Because of this style I can not ride many of the newer frames where the top tube is horizontally wider and thus the reason for my recent Giant Propel SL3 purchase.

This is correct.

On a long descent with little pedaling I would bend knees in for aero, but not during regular pedaling

Neutral is good, but what's neutral? It may be straight up and down for some but certainly not for all. It depends on individual body mechanics. A good fitter may be able to help establish this. Watch videos of accomplished riders and you'll see that most appear to be somewhat knocked-kneed while pedaling. I find I am too (knock-kneed, not accomplished :-)

OP- You will find that as a ride progresses, and your muscles tire, that your body will make certain small adjustments to utilize different small muscles for shorter times. These adjustments will include knees in, saddle shifting fore/aft, pushing/pulling on pedals, etc.

You can try and make a conscious effort to keep your knees in/striaght, or whatever, but at the business end of a monster ride, your body is going to put them where it needs to.

As stated above, you do not want lateral movement of the knees at the top of the pedal stroke. I use tapered cleat shims to eliminate that movement.

That's a bold statement.

I pedal with my knees slightly going in towards the top tube. Cleats positioned to allow this to happen on a natural basis.
Cheers.

I can't find the source, but I read somewhere that pedaling knees-in rather than at pedal width gives about the same aero advantage as an aero wheelset, but for free. Personally, I usually pedal with knees over pedals, but bring them in when pulling, bridging, descending, or just going hard on the flat. Main thing is not to have lateral knee motion, just up and down. I also ride slightly pigeon toed, which I think makes less strain when the knees are in.

Generally they're pretty straight (left/right), but I think it's much better for them to be a little in than a little out. And no, I don't do it intentionally, it just happens, probably follows from a good fitting bike.

Pass me sometime...... Quite the arrogant statement if you ask me.

to the OP:
No, yes, yes. I let them track as they will. I push down on the pedals, scrape back and lift in one fluid motion without care to my knee track.

When I started riding I tried to emulate the better riders around me. They all seemed to have pretty straight knees. Here and there some rider would have a weird jerk or something, due to hip/knee/something problems. So I focused on pedaling with my knees in a bit, especially when spinning. Not sure if that really helped or what but it's never hurt me and I have a relatively smooth pedal stroke.

Wasn't meant to be arrogant, just an anecdotal observation from nearly 40 years of racing and training. While there are surely a few exceptions out there, the vast majority of riders with such obvious issues in their form/technique tend to be inexperienced and poorly conditioned.

Maybe you should pass him anyway...

Go for it dude. Some arthritis in the hips causes my left knee to track a bit wide.

Admittedly it costs some watts in wind resistance, and I work on keeping my kees in.

However, my knee tracking wide didn;t keep me from a state TT championship.

Well good on you. Look, all I'm saying is that in most cases riders with visibly poor form are rarely very strong riders overall, especially when among groups of experienced riders. In one of the regular group rides I do which consists of a bunch of active cat 1 - 4 riders, there are frequently some new guys there trying to amp up their regimen. Some (not all by any means) of the newer riders look like new riders, fit, but their obviously poor form is indicative of their inexperience, and they're either barely hanging on or are completely dropped by the last few miles. Some continue to progress to the point that they are kicking my ass as well as many others, and what do you know, they look like completely different riders as well. I don't mean any of this as an insult toward newer riders, but it is what it is, and if you are out there riding a lot you know exactly what I mean.

Ya know what they say about assumptions? They make you seem like an uneducated crazy. I don't have great form, or fit. That said, I've worked with a very skilled physio/fitter who has done wonders for me, but by your criteria, I should be racing cat.5, and I'm kicking ass and Taking names in cat.2/3 right now.