Unless in the UK everything is completely different again (wouldn't be surprised at all, even with JAR-/EU-/EASA-OPS...) an aircraft under an AOC can only be operated commercially (or for training). If you want to undertake private flights, the aircraft has to be removed from the AOC first. This can be quite an act, bureaucracy wise, especially putting it back to the AOC again. Sometimes this can be necessary, e.g. if a self flying owner holds only a PPL or the aircraft needs to be operated to airfields that do not meet the factored runway criteria. These are the only occasions where you put a "G" in the flight plan. In my part of the world at least.
Then there are countries (like for example Germany), where fuel is not taxed for commercial flights but taxed for private flying. Sometimes the refulellers will ask you to see your flight plan together with the AOC. The tax inspectors will check everything available to them, including tech log and flight plans. If they find "G"s anywhere, they will reclaim taxes for these sectors. I know for sure because I've held the position of flight ops manager for over then years, before and after JAR OPS, and I have made my (expensive) mistakes just as anybody else.
But if in doubt, don't ask the internet, but _your_ principal at _your_ supervising autority (CAA). This is one short phonecall that may save your company thousands of pounds/euros/dollars!
Last edited by what next; 18th Aug 2012 at 11:52.