Ideally, it should never happen but we all know bumps do!
I try to never bump, unless of course some one hits me and does it on purpose.
I have in the pass, if I have hit some one and knocked them of the road I will wait to let them repass me.
I think this makes for better racing overall.
All this being said, it is sometimes hard to see who is around you or to know when the person in front of you is going to brake so I don't take the bumps and hits it the beginning or a race too hard.