I don't see why anyone would care even care if an old 550 carried a salvage title. If the frame is straight, front tubes aren't bent it's no less valuable than a clean title, or no title. I've seen valuable Ducati's that have slid down a track but never reported as an insurance claim, then later sold as a "clean title" bike. It's always buyer beware. Look for the obvious damage and make your buying decision on what you are willing to do to the bike to make it perform for your purposes.
I picked up my 76 550 with just a bill of sale and simply registered the bike in VT and then took my VT registration to the MA RMV and paid $25 for a plate and MA title. The cost to register in VT was $86 total and I had a plate within 5 days. Unless it's a special bike like the sand cast 750's I read about on this forum, I would never pay more for a SOHC titled bike than bill of sale bike, IMHO.