Most good slap players play in tune, but no bass player has perfect intonation and when you slap it becomes a wee bit harder still. There could be many reasons for this, not every slap player is formally trained, the left hand technique opens to slight imprecisions, but mostly I'd say stage levels are not always favourable on the bass player, plus the need for putting on a show sometimes makes it difficult to be spot on. However I do agree, Milt was the man, though I didn't like his slap tone on a few recordings after he switched to steels. Nevertheless, Milt was the best. He was a true legend.