With the majority of the songs I play very close to the original, especially the Cliff Burton basslines! I usually improvise a tiny bit on mellow sections like the interlude in Master Of Puppets though with a bit of a chorus effect (nothing distasteful though!)
Some tracks I feel the bass could offer a little more in locking the drums as guitar together, and grooving, rather than just playing root notes on the 1st and 2nd beats... So I do alter them a bit. Partly to improve the overall sound and feel, partly so that I don't get bored (and that I don't look rubbish/lazy )