as long as you have enough power to move the needle to the calibrate line, and not too much to blow the meter, you can go either way.
aside from re-calibrating the meter to adjust to the different power, your power level shouldn't matter. from what I've seen, a good match will be a good match and power increases and decreases don't change it.
of course if you key up with higher power and a bad match it would be more strain on the final(s), but for the couple of seconds you need to adjust and read the meter there shouldn't be a problem.