The common perception is that flaps always both give and take away but there is a little more to it. They give by increasing lift at the cost of increased induced drag; however, you have to consider total drag. Yes, lift always causes induced drag; however, this is irrelevant at or near the stall limit since flow separation (the beginning of a wing stall) generates even more drag than the induced drag from the flaps. This is what causes many people to get "parked" at the top of the loop, they bring the airplane up and over, begin to stall (the airflow begins to separate from the wing), the flow separation drastically increases drag and the airplane just sort of stops. By dropping flaps, you increase the ability of the wing to continue to develop lift with less increase in total drag so you maintain more speed and can more easily pitch the nose over. This is also why some planes have a "maneuvering flaps" setting and are usually fought with the flaps down one notch. The P38 and P51 are a couple good examples where the flap limits are high and you may want to just leave them at one notch where the airplane's average drag during a hard maneuvering fight is lower and the turn rate and overall ability to sustain e is improved. Also, this applies when you're near the stall limit, you should always raise the flaps otherwise.
Generally speaking, if you're coming over the top at anything less than corner velocity use at least one notch of flaps (provided they'll come down, remember maximum flap speeds differ for each of the airplanes.) Think of it as whether you're riding the edge of stall (stall limited) or riding the edge of blackout (G limited). If you're stall limited use a notch of flaps (again, if they'll come down at your speed). The slower you are, the more flaps you can use (if you need more) but then get them back up again as soon as your nose is below the horizon and you're accelerating. Even on airplanes without intermittant flap settings such as Spits and Hurri's you can still start the flaps down as you get near the top and then toggle them right back up again as your nose goes below the horizon (their flaps usually move so slowly that they never get below one-quarter to half-flaps before they're coming back up again.)
Last, it should be obvious that flaps need to be mapped to your stick so do that if your stick has the buttons. Most good pilots will be constantly working those puppies up and down throughout a fight and you can't do that well if you're constantly searching for a keyboard button.