Your 70-200 is fast at 2.8... however I wouldn't borderline shutter speeds.... there is usually constant motion in performing arts...
While aiming for the eye, I'd determine needed aperture for the depth of field desired to get the subjects in focus...that includes instruments, arms, legs, elbows, noses and chins, etc. and then determine what speed will compliment the needed aperture...
Winkie, I'll let someone who knows the exact terminology for the shutter speed formula explain it to you...
However, here is an off the cuff explanation.. an example of minimum shutter speed would be: a 50mm prime lens, hand held on a full frame camera would require a minimum of 1/50th of a second... a 100mm prime lens would require 1/100th... Now with a 1.6 crop camera, the minimum shutter speed would need to be proportionate at 1/80th and 1/160th of a sec...
however, it is my belief that since a zoom lens is long, than I'm of the opinion that on my 70-200mm, I would want a minimum of 1/200th on a full crop and 1/320th on the 1.6 crop camera... I'm also not comfortable with my 24-70mm f2.8 lens because it zooms out as it gets wider. so to be safe I would want at least a 1/70th on a FF camera and at least 1/112th of a second on a 1.6 crop camera... Since the shutter speeds are in specific increments, it would be wise to go to the next higher shutter speed...