I don't try too hard to keep things all original, as I see that as impractical, and my bikes aren't super-special grail bikes.

[edit]
It depends a lot on the bike.

With my Bianchi Megatube, I would always keep that period-correct: e.g., I wouldn't stick a more modern (outboard cups) BB on it. 8-speed Campy Record and a square taper BB is what it should have, and that's just fine.

But the entry-level aluminium M Alloy Pro, or the steel Colnago, sure why not put whatever on it?