It depends on the gun. If you're talking about AEGs, then the blowback effect can barely be felt. The part that's moving is so light it really has no perceivable effect.
On a good GBBr with strong recoil, it will have some effect on your aim. But it's not like we're talking real steel recoil here, so the effects are pretty negligable when it comes to tagging a man-size target at 50-100ft. It's easily compensated for. Wind and air currents will still play a much bigger part in your accuracy than the recoil will..