Given that you can get a good off-the-peg fixed gear bike for under $400 now, it would be a real shame to see a sweet old Pinarello get the conversion treatment. The other argument is that as long as you don't do anything stupid like shave off the braze-ons or "rattle-can" the thing black, then anything you do can be easily undone.

I tend to think of fixed gear bikes for the rode as sort of lock-and-leave things, the kind of stuff that you bang around on and not worry too much about. Even if you converted that, I'd still be afraid of nicking the paint or worse, so I'd definitely keep it as is, and use it on long weekend rides or whatever suites your lifestyle.