Preflop all-in in deep with a premium hand is pointless, because if there is no other call able hand, you only win the blinds, which is a wasted opportunity. Of course, if the table is very loose and go for the flapping and allin call., then preflop allin makes sense, but this is a rare situation even at the freeroll level.
In any case, it is worth varying the game, regardless of the bet size. At lower stakes, it's much less common, but sometimes it's worth playing AA slowly.