I commuted most days last winter and found that there were surprisingly few days where this was a concern. I rode mainly on through routes, with high traffic volumes, the roads have usually been salted and bare off quickly. With snow banks the roads are a little narrower, therefore you may have to "take the lane" more frequently in the winter. If you allow traffic to squeeze you over into the slush/ice/crud along the snowbanks, you could indeed experience the problem you described.
My worst spills last winter were on the road in front of my house (snow on top of glare ice) and crossing a sidewalk through a parking lot shortcut (freezing drizzle on sidewalk, not on road). I had one stormy ride where the roads weren't done very well, built up ice and slush on the roads combined with slicks makes for very tricky handling. I had to ride very slowly, but traffic was sparse and moving very slowly anyway - I stayed to the side of the road, trying to ensure I would fall to the curb if I did capsize.
Like regular weather commuting, being visible and intelligent, defensive riding are the most important things to think about.