When you're working with rounded objects whose edges have been smoothed, it's sometimes hard to make things look good. That's because curved surfaces don't automatically produce a profile edge that helps to differentiate them from the background. You can see what I'm talking about in the images that follow; notice the (what I consider to be) unsatisfying outline of each of the rounded objects below?
Without Profiles turned on, rounded objects don't stand out.
Turning on Profiles in the Styles dialog box produces a completely different result. At a Profiles setting of 2 pixels, perimeter edges become clearly visible. They're a little chunky, though—and that's not always the effect I'm aiming for.
Profiles that are 2 pixels thick often look too bold and cartoony.
Dialing down Profiles to 1 pixel solves the problem (see below).
Using a Profile thickness of 1 pixel makes rounded objects pop out from the background.
While this trick might seem obvious, it actually took eight years to soak into my brain. I never understood the benefit of setting my profile thickness to a single pixel. After all, edges are already that thickness—why spend the computer cycles to draw them again? Now I know. I thought others might benefit from my epiphany, embarrassingly late though it is.
It's worth mentioning that telling SketchUp to draw Profiles can slow things down considerably if your model's pushing the limits of your polygon budget. I only switch Profiles on when I need them.