as LYNX stated, it is all a personal preference really.....
I prefer to take any plane in the game I am flying and trim it out in level cruise speed....for most planes this is in the 275 IAS ( white needle ) to 325 or 350 IAS mph.........
only time I consider using the combat trim is in planes that have lost 1/2 a wing or an aileron or has become damaged in one aspect or another.......or if I am in a down and dirty dogfight with an opponent flying the same plane type....that knows what they are doing...
I find I can sqeeze more performance from a plane flying it trimmed out like this rather than fighting the "nose-up pitch" that combat trim seems to induce, especially when slow.
If you are using combat trim, and flying slow, and using flaps, combat trim can become your worst enemy......combat trim and flaps will be fighting each other in a sense.
now when you are coming over the top of a loop and start to nose down / dive you will be fighting combat trim as well, and will take longer to pick up speed
where as if you are trimmed out nuetral ( or what I call trimmed to level cruise speed for particular plane type ) you can gain a bit more speed faster and not be fighting the reaction of combat trim.......
but as mentioned above, it really is Different strokes for different folks.......
hope this helps.......
edit: as for flaps giving a nose-up bump and people who have problems with the bouncing nose syndrome.......I'd suggest trimming out to have a slight nose-dwon aspect, or in other words to where you have to maintain a slight easy pull on the stick when flying level..this will help with the bumping nose up , when slow and using flaps, and will also help when lining up / saddling up for a shot, you will have better control of holding bore site on your target and not be bouncing all around fighting the trim settings of combat trim.....
