It depends on what you're doing. I know that when I'm shooting pop cans at mid-range (300-ish yards), with my 24x it's great. But shooting 300m BR, I'll shoot much better with my $400 Weaver T-36, than the 24X that cost 3 times as much. (And in those events, others with higher power BR scopes have an advantage.)
Shooting from bags, you should have no issues with 32x and up, or your form has a flaw.
In general, the higher the magnification, the finer aim you can take, making you more consistent. If you check 6mmBR.com and look at the winning guns in F-Class, they're pretty much all higher magnification.