I built a fantastic commuter bike a little while back, and among my component choices was one of those "new-fangled" outboard bottom brackets...Shimano Hollowtech II. (I like the concept, and the capless look on the right crank.)
I've worked with square taper cranks before, and when I've installed those I generally try to get everything as tight as possible, without being stupid and breaking/shearing anything. Nothing can ruin a ride like a loose crank arm!
When I started to put my new crank on, I...um...didn't read the directions very carefully. Why should I? It was so elegantly simple in concept and execution. I simply checked that the spacers were right, and then off to the races.
I tightened everything down real nice and tight, starting with the dustcap.
Fast forward many months, and there's a terrible creaking from my bottom bracket every time and apply real power to the left pedal. I troubleshoot everything...I lube my pedals, I try different pedals, I check the bolts clamping my saddle. The creaking is still there. If experience tells me anything, it's a loose crankarm, or worse yet a loose bottom bracket.
So I take it apart and lube everything, and this time make sure it's really, really tight. And oh lordy if that creaking isn't worse. Finally I cave and go to the Park Tool website.
Apparently, if you tighten down that dustcap too much, you can side load the bearings. This, by the way, is a Very Bad Thing.
So I started again, and this time used a more delicate touch with that dustcap. Sure enough...creaking gone.
This is my cautionary tale. Truisms from one type of system don't always carry over...and reading directions never hurt even a decent mechanic. And if you're going to get one of these systems, it really is great!
Just don't overtighten that dustcap.