I would do it at no charge if something simple like moving a receptacle or switch a few feet to accomodate a doorway, etc. No real cost in this if told ahead. I would still get a signed change order and mark it "no charge". Just in case the specs change again & the GC gets a memory lapse.
Anything bigger, or changing work already done, I would get a signed change order before doing the work. Again, memories are conveniently deceiving. Keep your boss in the loop at all times, unless you are the boss. If so, keep your office manager involved, to keep all your official copies up to date. Don't rely solely on field copies, as they can be easily lost or rained on, etc. If someone doesn't want to sign a change order, he is planning on stiffing you. "No problem, we always pay for these things" will turn into "I don't recall that" and for sure won't mean anything when you deal later with an out of town/out of state office manager with no record of the request. Your only leverage is before the work is done.