I guess I've assumed that a bike would prefer to cross in front of me, someone who has acknowledged their presence and also happens to be blocking the lane behind me, rather than wait and take their chances with the next guy who may not be looking. Perhaps I'll reconsider that in the future. I still think I will do this in at least some circumstances, like when I see kids on bikes wanting to cross. In the case of kids I make a point to check all the traffic before waving them on and basically try to act as another set of eyes for them while they're crossing.