Mixing the ailerons to rudder really is a matter of opinion, mine is learn to fly with both sticks and then if you want do some mixing.
The specific question was about the Extra 300. I say fly it first and then see if you want some rudder mixed in.
Aside from V tails and flying wings ect... I can't think of alot of planes that can't be flown without mixing.
What I am saying is, your Extra 300 can be safely maidened witout Rudder Aileron mixing "IF" you have anough skill to maiden an airplane like that, if you don't then mixing won't make alot of difference.