My opinion: If promoting to more queens gives you a faster checkmate, go for it. If it's just to spite your opponent, it's probably rude.
How can you spite your opponent by promoting more queens than needed? All he/she has to do is resign and there is no 'spite' to begin with. Catch my drift?
And firebrand, I get your point as well, I typically resign once I see there is no hope. but if I'm in a lost game, and all of a sudden my opponent ignores my king and goes unnecessarily grabbing material and trying to promote when the game should be done in 5 moves or less, then I won't resign, I will try to see if the patzer I'm playing ends up stalemating me for a draw.
Or on a similar note, if it was a good hard fought game, and my opponent surprises me with a good checkmating pattern (like one involving a sacrifice or two) I will go ahead & play it out just to be sporting and let him see the final mate he earned from clever play.
Depends on the situation. If you have a rook or better and your opponent doesn't, then you already have sufficient material. But if in same situation, your pawn is already advanced to the 6th & the king is already on his back rank, then go for it if it makes it easier. But if all you have is a minor piece (or even 2) then go ahead & promote, as mating with a single knight or bishop is pretty well impossible, and takes about 20 - 30 moves with a king & bishop pair. Knight & bishop mate is excruciating, as you first have to drive the king into the cornor opposite your bishop color, and then push him from there to the corner that matches your bishop, which will generally make the game run 100 moves or so. (half that just trying to herd the king)
Rude is when you have a rook & bishop and all he has is 2 pawns, and instead of just going for a mate, you run around and take all his pawns and make 3 queens for yourself. Such is the sort of sportsmanship one would expect from a 9 year old. The goal is mate, get 'er done.