The only way a quality brake will negatively effect precision (group size) is by changing the harmonics of the barrel so that whatever load you developed without a brake doesn't shoot as well with the brake. Just tweak your load, if necessary. Like Bcbrad said, a good brake ( I use Insite Heathens) will greatly reduce muzzle rise and make it easier to spot/follow your own trace. Is a brake "necessary'? Depends on what you're trying to achieve. It won't make your rifle more mechanically precise (as in shrink groups) but it will let you spot trace easier which goes a big way to helping you be more accurate (ie put more rounds on target).