Round corners render effect?

[Fletch]

a beveled edge (45 degree angle) will not significantly add polys to your SU model... but will confound you when you try to modify the same model later.

it does not add to polys, correct, but it may add a bit onto render times... have never tested that personally.

[Pixero]

That's the beauty of such an effect. That you DON'T have to model it on every single piece of geometry. Just apply the "bevel" render effect and you're done. And it looks so much better than without.
Modelling bevels also can complicate it if you want to edit the model later.