The quick answer: listen to your monitors!

If you're too low, the front element of your performing ensemble will be masking/overpowering the players in the rear. If so, raise the mics. (Also, if too low, you may get a lot of floor reflections that could muddy things up a bit.)

If you're too high, the reverse may be true (i.e. the rear of the ensemble may be too pronounced). Also, you may notice a change in the timbre of many instruments, esp. strings.

Cheers,
Mike

That's a great tip! However, most of the time I have another dilemma, which is I only have my portable DAT and a pair of headphone to monitor the source. In that case, does the theory still apply? Thank you!

~D.