You don't need many Dedicated(fixed) mixes.
Flaperon is handy---it allows separate ail. servos (which in turn allows easy set-up of individual throws up and down, left and right). The actual flap function part of flaperon is usually disabled for pattern..
Dual elevator("Mate" in some sets) is also useful----but can be arranged as elevator-to-another (slave) channel if not available.
Aileron Differential is sometimes needed--if it's available as a function it can save a little time,but you can do the same thing with end-point adjust(travel adjust) if you have flaperon and separate aileron servos.
Go for as many "free" mixes as you can get (user defined)---You'll almost certainly need Rudder-to-aileron, and Rudder -to-elevator.
You might need throttle -to- elevator(Straight downlines), possibly throttle to aileron(to catch slight roll at low speed).
Other mixes are less common, but the more free mixes you have available, the more little fixes can be done.

In the more advanced classes, most flyers will require "flight condition"(s) .This means changing a few settings simultaneously on one switch e.g. more aileron,etc., for snaps, or more rudder for rolling loops or rolling circles for instance.
There are lots of reasons why PCM is used almost exclusively, but the most obvious one is that glitches and accurate manouevres don't go together.