I have the same rule as others have posted. I won't customise anything unless I have a complete set that is un-customised that contains whatever items I customise. So far I've only customised a few figures though, and I usually buy sets or DS orders with the intention to customise the figures within them.
Of course, I switch around parts like accessories, weapons, and armour without considering the completeness of sets though, since this does no irreversible customising.