On my 2006 and my 2007 SS800 there is a sticker on the frame right below the tank where it list the stock size and the optional replacment size. The dealer selling you the tires will know if there isn't any sticker.
Here's my take on the wider rear tire topic. I happen to love the look of a wider rear tire on most any bike.
I replaced the rear wheel on my S4RS with a 6.0 from a 5.5 so I could make the change from a 180 to a 190 appropriately. The point being; if you don't increase the wheel size you're really not going to notice a difference in width from a visual stand point. The lip of on both tires contact the rim at the same width, the extra rubber on the 170 will just be pushed upwards, hence creating the steeper profile. So the 170 really won't look wider as it will look rounder or have a higher arc.
Finally, without increasing the wheel size you will probably change the handling of the bike in a negative way.
I thank all of you very much for the great feedback on my question. You have all made great points and I will most definitely stick with the stock tire size unless I do decide to increase the width of my wheel.
A little late probably, but the 620 will most likely have a 4.5" rim like my 900CR. A big difference from the 5.5" on most other ducs, so you have to stick with the 160 size for all the reasons stated above. The 170 would probably even wear faster than the 160, due to improper profile/contact patch.
The Cr has the same swingarm as the SS, so a 5.5" rim will go right on. I do not know about the 620 though.