IMO...
It depends on what you want to accomplish. When I create IRs, I will setup a mic, and capture some playing, review the recording, and when something piques my interest, I repatch for an IR capture and record a few sweeps. I set things so as to have the levels as close to one another as possible. Sometimes I will increase the power amp level a little for some tonal variation.
Levels will provide difference, and for different reasons. Mic technique, cab placement, recording environment, etc are all vital contributors as well. For a basic, close-mic capture, just keep it loud enough to stay above the noise floor (very, very simplified suggestion).