My first question- are we really talking about a rifle that can dot an "i" at 300 yards? There's precious few of those in the AR platform, so before we really talk about how much it might affect accuracy, I wonder just how accurate the thing is to begin with. A 16" barrel is not usually the one chosen for long range shooting- they can indeed be accurate, but they aren't often the first pick for that.
If your rifle is sub-MOA and you want to keep it that way, mount it to the handguard; it's the only way to be sure you don't affect anything. If it's a 2-3 MOA rifle, then the bipod on the gas block won't affect anything. If you want the best of both worlds, get a quick detach handguard mount for the bipod
and put it on when you want to use it.