Original finish is always best, but especially so on a really rare/popular bike. If a chipped, scraped, rusty bike's not particularly sought after, has already been repainted, or is just really ugly, I think it's OK to re-paint. BUT, even perfectly re-painted, it's then worth less to a collector than one with good or better original finish.

One frame I just asked mswantak to make decals for ('70-ish Rickert, in this case), had already been buzz-bombed (poorly) by a previous owner, and had no original decals. The frame's straight, and in good shape other than that ugly, runny orange paint.
I figure it's a perfect candidate for a restoration, but didn't want to spend the $400++ to have a pro paint it (It's a Rickert, not a Masi). This will be a pretty, regular rider when finished, so I wanted it to look nice, but not for bike shows. When I'm done with it, unless right next to an original paint Rickert, you won't know it's not original. To anyone who knows the finish isn't original though, it won't be worth as much as one with original paint and some patina.