I would never resign a game where the expected outcome is a win on time for me. Of course offering a draw is a gracious thing to do and could be considered good sportmanship, I usually only do this if I have been playing several games with this opponent and maybe chatted a bit.
I would also offer a draw if the position of the game is impossible to ruin, even in blitz, for either side, which is quite different from a "dead draw" in tournament play. I.e. something like KB vs KB, K+a-pawn vs K with the defending king in front of the pawn, etc. It doesn't make much sense to me to find out who can purely physically make irrelevant moves faster.
But in dead drawn positions in which my opponent still has to react to my moves, I would certainly play on. Often you have a situation in which you can instigate a pawn break as soon as your opponent is reduced to making premoves. This pawn break would lose if he were still able to react to it, but, alas, he used up all his time parrying my earlier threats.
That's just what blitz is about, you exchange time for quality moves. If you can't stand to be flagged, play with increment.