I use a CH throttle quad (which actually has 6 levers, not 4).
On it I have mapped aileron and elevator trim. It really took a while to get used to, but now that I am used to it, I think it's very helpful. I can trim hands-off sometimes, but generally use auto pilot if I want to do that. I still use combat trim, though, at times. In a fight where you blow speed or follow a diving con, your speed changes and constantly adjusting the trim is a chore. For many fights you can set a "medium" speed trim and keep it on that for the fight.
I tried initially mapping rudder trim. It's not as helpful. Every time you bank the ball rolls left or right in the slip indicator. I've got rudder pedals and it's a bit more practical to use them to try centering. Plus, if you map rudder trim you don't get the benefit from toggling combat trim on and off. It won't stay at the last trim for that speed when you turn it off. Instead it jumps back to the axis input.
I've recently started playing with rudder trim again on my fighterstick's throttle axis. Not very convenient, but I haven't decided to get rid of it or not yet.
I have mine for my left hand. From the left most lever I have it mapped as such:
1) aileron (push = roll right, pull = roll left)
2) elevator (push = nose down, etc)
3) zoom FOV (push = higher zoom)
4) rpm
5) engine 1
6) engine 2
I started out having RPM 1 and 2. Makes no difference, but it's fun. I sacrificed one of my RPMs for zoom FOV. It's a nice thing to be able to control.