I put my commuter bike on the bus frequently as I often don't have time to ride my full 46 mile round trip commute (or am feeling lazy!). I've learned to quickly discern the condition of the rack, particularly its restraining arm, before loading the bike. Usually they're well maintained, but if the arm has a lot of play or stickiness to it, I'll ask the driver if I can use another slot on the rack. So far, they haven't refused this request.

Edit: Reason for asking the driver is most of them want you to use the outermost rack if no other bikes are present.

I'm an enthusiast about multi-mode commuting, but I wouldn't put a really nice bike on one of these racks- especially one with thin, light rims. I can't speak for other transit agencies, but I suspect that many have the same "use strictly at your own risk" policy with respect to bikes on racks that Metro Transit has in the Seattle area.