Yeah it's happened with me a small number of times.
While there are almost always a few cosmetic changes I would make for my own 'personal' tastes, there has definitely been a few occasions with 1 or 2 track digi-releases where I have simply written back to the client to say "These sound great and are really not worth messing with."
They generally appreciate the honesty, I don't charge unless I actually need to produce a physical master, and I find this gets me more work from those labels rather than less.
I have never in my life heard of any project that needed NO mastering. I've gotten projects that didn't need any processing, but I still had to put the songs in order, set the gaps and make the disk. I guess what I'm saying is, not needing sweetening is not synonymous with not being mastered. ALL projects are mastered if they've been replicated. All of them, no exception. Even if all the engineer did was set P/Q codes, it's been mastered.
I thought The Flaming Lips were infamous for bypassing the mastering process... which isn't exactly the same thing as saying their work didn't need mastering... then again, somebody had to sequence the tunes, etc. and that of course is part of mastering. I guess what was meant is that they disallow any processing on their stuff ====