its perfectly fair to use whatever ball u want 4 freestyle. its "freestyle" there aint no rules. its still freestlye with a size 3 ball, its a matter of opinion. ALSO
some tricks can be EASIER with a size 5. like headstall, shinstall , actually EVERY STALL cos the ball is easier to balance. i use a size 4 and im happy with it. i will buy a monta ball soon tho, they are good