I set my trim wheels to fly straight/level at about 300. I have ACT off by default, but have a button mapped so I can easily go back and forth with it. I do my "general" flying with ACT on, but turn it off when fighting, landing, damaged, etc.
I never adjust it during a fight, apart from toggling it on/off as necessary. If an opponent dives out and I'm going to pursue, I'll generally switch ACT back on, but if/when he breaks/reverses I turn it back off. I'll also make sure I have it off if I'm roping, stalling, etc. I don't necessarily switch it off immediately when I decide to fight. I may leave it on if the passes are gentle, fast, etc. When I decide to start getting serious the ACT gets turned off.
I'm not sure how much advantage that gives me, I just like the way the plane behaves better. My squadie never turns his ACT off (except if damaged or landing), and doesn't have any issues either...
I have it off by default mainly because coming out of auto-climb, auto-level, etc I prefer to be in manual mode.