Rule of thumb: Always expect to have to putt, and a concession by the opponent is a bonus.
Rule of thumb: If a putt isn't conceded and is then missed, we now know WHY it wasn't conceded.
As others have said, if the guy is mad at YOU because he missed a putt, he needs to go home and think about it. Not all 3' putts are created equal, both because of the greens, and because of pressure and the meaning of the putt.
Each of the majors has been lost at least once by missed putts of LESS than 3'. Scott Hoch and Doug Sanders leap to mind. There wasn't a concession possibility there of course, but the idea that somebody OWES me a concession because I was able to get within 3' of the hole is begging, not golf.