Funny, I was thinking about this yesterday. Someone said to me - 'Didn't The Beatles ever record any crap? There must be tons of other songs in the vaults'. Well, there isn't really, is there? I mean, original songs, unreleased by them when they finished added up to a mere handful. I think they knew when they were writing them whether they were of a high enough standard to spend time recording. So they must have practiced individual control. So the old thing about there having to be 5 John songs, 5 Paul, a couple of George, and one for Ringo to sing doesn't really stand up. Those were just the songs they had. If Paul and John had had a few more songs between them, I doubt that poor old george would have got much of a look in. Even when they did have far too many songs they released a double album.
I'm sure George Martin had a lot to do with their first few singles. He definitely chose Love Mr Do, and Please Please Me. After that I'm not sure.