speed? pretty much everything is better on the bike with a bit of speed. momentum is what keeps the bike upright. I'd say 8-10mph minimum.

now, even though I teach bunny hops in my clinics, I don't recommend them for group rides or racing, especially if you're in a tight pack or paceline. timing is difficult and unless you really perfect the skill the chance of landing off-line (ie to one side or the other) or directly on the obstacle you're trying to avoid, is pretty high. I don't think this is a safe skill for group riding.

just my opinion.

second reason not to bunny hop? I've cracked hubs (Chris King), broken spokes (DT Swiss and Wheelsmith), and cracked rims (Velocity). granted, I put a lot of stress on my bikes, and I ride on and off curbs in addition to hopping. but the reality is that it can put a lot of stress on your wheels (and potentially other parts of your bike).

again, just my opinion.