that's kind of a myth; they all use the same crystal oscillator tech as a digital watch, so A really does = 440 on all of them.Were I gigging, I'd want everyone on the same brand of tuner.
some tuners are more precise than others, but that's more about how wide the window of what they consider "in-tune" is or how well they track; a boss TU-2 has a +/- 3 cents error window*, which means there's a 6 cents wide margin of pitch ranges that it will call as being "in tune", but the "center" of that window is still dead-on A=440.
*this was by design; the TU-2 was made to be strictly a stage tuner, so the wide error window allowed you to quickly get "close enough" so you could start the next song already.