round corners subdisns to no subdivs

SubD is a good method, but depending on what you want to do with the model afterwards, you can also spend some time to clean up some unnecessary edge loops from the subdivided model. In other words, SubD will generate edge loops in places where it's not needed for the smoothing process, just by nature it will split all polygons into 4 each iteration. So, once you get the level of rounding you need, it's quite simple in most cases to reduce polycount dramatically simply by removing edge loops from flat areas that aren't contributing to the curvature.

There is no simple answer to this question since you're describing very (very) general situation. Actually, on such question you can't get the answer but just another question.
Firstly - what is that model for?
Is it game asset? VR model? Just for rendering? 3D printing?
For games or VR (or something similar) you can care about poly count.
Jjust for rendering - you can care but you don't have to care.
For 3D printing... Bring it on!

So... First counter-question is: Do you care about poly count? And: Why?