Op -- dedicated winter wheel/tires are the way to go..Or do what I did and have a 325xi waiting in the wings for winter.
Anyways, my main point is do your homework on the right tire. All winter tires are not the same.
There are snow/ice tires and winter performance tires. If you were living in Fargo, you'd probably need to go snow and ice tires for the extreme snow amounts. And then, you'd live to regret it because they are like driving on rubber bands.
Winter performance tires on the other hand will not give the same traction as snow/ice but they are good enough - especially for Pittsburgh. Blizzak for example has a WS series and an LM series. The WS are snow/ice, the LM is their winter performance tires.
I've had the LM's on a # of my rwd cars over the years and really liked them. For the 325xi we have now, I put on Dunlop Winter Sport 3d's - loved them just as much
Ask for Mason @ Tirerack...He'll get you hooked-up with the right stuff
Current Ride: 2013\28xi\Deep Sea Blue\Oyster Nevada Leather\Fineline Sienna Trim\Sport Activity \Cold Weather\Premium\Comfort Access\Navigation\Panoramic Moon Roof\BMW Assist
Previous Rides: E92 M3, E70 X5 35d, E71 X6 50i, E71 X6 35i,E60 550i, E46 M3 Cabriolet, E39 540i, E36 328i