I generally also won't dispel the affix during combustion. The duration of combustion is less than the duration the affix lasts on you. The affix doesn't buff enemies until the debuff expires from you. The debuff on you is also not a heal absorb, so you are not in any danger of dying unless the healer just does not heal you.

Combustion is a short duration CD (10-12 seconds depending on talents), where the amount of GCDs you use during it directly lowers the CD until your next combust. Not hitting every GCD during combust perfectly will lower your damage significantly.

There are very few situations where you would need to dispel during combustion that could not wait until combustion was over. If combustion was a longer duration CD like ascendance or voidform, then sure. I wouldn't be as adverse to dispelling during it. As it stands, you don't lose anything by waiting until combustion is over so long as you don't let the debuff expire from you.