Having a little knee pain on the left side. Seat height seems to be ok (within normal angles--146 max, 73 min), so I'm thinking it may be cleat-related.

I've been playing with the toe-in / toe-out adjustments, trying to see if it helps. I'm thinking maybe I need to be a little toe out, to mirror the natural resting position of my feet.

How do your feet line up when you are clipped in? Are they straight ahead? A little toe-in? A little toe-out?

I definitely ride toe-out. It may also be how you pedal. Make sure your knees are not bowing out or fluctuating in your cadence.

Somewhat toe out for me.

Toe in or toe out is totally dependant on Q for me. Narrow is better which causes toe in however my natural walking stance is toe out. Varus / Valgus wedging will also change what is comfortable

I'm a little bit toe-out.

Also check your cleat position on the shoe (side-to-side).

What sort of cleats?

If they're Shimano SPD-SL I'm assuming you're onto the yellow (floating) cleats already...if not try those first. I think they give about 6deg of float (someone correct me here). But that should be plenty to correct a toe in/toe out issue. If not I'd be looking elsewhere...

Take a couple of steps and stop to look at positioning.
Thats usually your resting position. Check which way your
feet look. +1 on the yellow Shimano cleats. Alot of float there.

IS it both knees? Have you ever been fitted on your bike?

I had a few knee issues only to discover when I went in for a fitting that one of my legs was longer than the other.
Had my cleats on the longer leg adjusted, and it solved the problem.

If you are using Keo, I much prefer the red cleats. Lots of knee saving float.

Thanks for all the comments. I am using speedplay zero cleats, and the pain was in both knees, but was worse in my left knee.

I tried adjusting them to be a little more toe-out, and that seemed to help. I am going to go even a little more toe-out to see what that does. To accomplish this, though, I think I'll need to move the cleat slightly inward on the shoe.

Your foot should be in its natural position, whether that is toe-in, toe-out, or straight.

In the good old days of slotted cleats, you would just put the cleats on with the screw finger-tight, go for a spin around the block, and come home and tighten them up. Worked PERFECTLY!

Now you can't do that because the unclipping process will make the cleat move.