In general, any kind of operational checkout on a piece of equipment is done with the piece being tested at as close to operating conditions as possible. Specifically, in this case, an internal combustion engine does not ‘seal’ up (PTWC, etc) until it’s at full operating temp, and that is when any kind of check should be completed.
I do agree with the lumberjack that something is slightly different with that cylinder when it’s not up to full temp (keep it mind it’s read from 10-15psi lower than the others, not 50 or anything), but I’m not concerned at all. I’ll continue to beat the shit out of the car and check the compression every plug change and update the thread if anything changes.