R.J - it depends, as always, on company SOPs. BA USED to forbid it on the 737 (2004) - don't know now, and Astraeus procedures (737) do not include it.
Edit following mach79 - didn't notice R.J's error (RTFQ!)
T'was cruise MAC entry to which I referred, of course. Who knows? Maybe R.J's company do actually calculate cruise MAC before departure, but as you rightly say, PerfInit requires MACTOM.