Yes, leeneia, the tendency for vocalists especially to shortcut the held notes is real. It drives me crazy when playing for hymn singers. I always have the metronomic thing going in my mind's ear while holding a note. Probably comes from playing while marching - you want to start the next phrase on the correct foot!
I also find there's a counter-intuitive tendency for some lead instrument players to SPEED UP in the hard parts. Seems to be a subconscious temptation to slur over the tricky bits rather than articulate them carefully and accurately. I have to fight this tendency myself on the pipes, not so much on guitar or keyboards.
It doesn't hurt (once you know what tempo you're after) for the starter to check his or her watch discreetly before launching a number. I find this helps me keep from starting at too fast a clip sometimes.
You would think a "good" drummer would be metronomic by definition, but being human they are subject to the same sensory illusions we all are. It definitely takes practice and not every player cares.
It does help to keep an eye on the dancers. Their whole-body involvement in every beat (even the held notes) tends to keep them more consistent. If you're starting to get to the strong beats while they are still in the air... back it of a bit.
-Glenn