ORIGINAL: britbrat
I don't use it on pattern planes -- or any other aircraft with symetrical airfoils -- I use the rudder to counter adverse yaw (if any). You are correct that it negatively impacts inverted flight.
Differential on many pattern planes is required or your model will not roll axially.....
I have never noticed a negative effect on inverted flight, or vertical rolling manuvers...in fact if you dont use it on some models the heading deviation during 45 degree/vertical climbing rolls is very significant.
No theoretical reason as to why, just 1000's of flights of experience doing it.....