I have heard that it happens and heard that there can be TWO bumps...one for a nomination and another for a win. I imagine it all depends on how well the agent negotiates for their client and how likely anybody thinks it is that a nomination and/or win might happen.
Coincidentally, I once worked with a multi-award winning actress (who has a Tony) and this very subject came up. She said that she didn't go in for that kind of thing at all because the work shouldn't be about winning awards. The work is the work, regardless of awards.