Is it bad form to go for a lengthy but losing endgame that you suspect will be winning on time? In blitz games, I sometimes have to choose between sharper positions that I suspect will put me at some slight positional advantage and positions easily played by my side but difficult for my opponent which, if they play correctly, they should win.  

For example, I might have the option to exchange down to a queen versus rook endgame where I have a 1-minute time advantage or go for a rook and bishop versus rook endgame where I have the extra bishop but where I might have to use up a lot of time to get there.  Given that choice it seems to me that holding a rook versus queen endgame is relatively easy if my opponent is under time pressure (assuming the endgame is not easily converted into the Philidor position) but winning the bishop endgame is less certain.  

That said, when I start sacrificing material for no real advantage except to temporarily solve my opponent's most recent attack, it seems to me that my opponent is likely to get irritated that I am trying just to win on time.  I'm not sure what the appropriate etiquette is in such situations regarding play, however.
All is fair in love, war, and blitz (at least with regards to winning on time, and short of outright cheating).

In blitz, time is a major factor in the game, and it is fine to try and win on time in any situation.

If you used too much time, and your opponent thinks he can flag you, there is nothing wrong with that. It is part of the game.

I have seen Grandmasters check with a queen right next to the opposing king that can be taken for free to win on time, just because it requires that split second to realize what happened.

P.S. If someone does not like being flagged, they can always play with an increment.