On the IS300, the stock 16" wheels (with higher profile tires) cost the same as the 17" with 45 series performance tires.
Yes, the 16" wheel package looks less agressive.
1> You can get many more winter tires which grip better in snow/mud/ice in the higher profile types.
2> You have much less chance of scraping your wheels on a curb.
3> You have a little extra "tire cushion" to smooth out the bumps on really bad roads.
4> Your higher profile tires will probably last longer (harder compound).
Since it never really snows around here, and California Lexus dealers are getting pretty much 100% 17" wheels, my choice was easy...