Is it unsportsmanlike to underpromote a pawn to improve my chances? Sometimes I reach a won endgame where underpromoting my pawn will result in a slower but easier final checkmate.
Old example, easiest to understand: I was far better practiced at K-R vs K than K-Q vs K so I promoted a lone pawn to a rook. Improvising K-Q vs K would have had too high of stalemate chance for my taste.
Recent example: I deliberately underpromoted a pawn to a rook rather than a queen while having a queen. This makes mate in 9 moves worst case rather than 8 and again reduces the accidental stalemate odds dramatically.
I barely care about my opponent not resigning when he should. Primary reason: I once played out a losing game, sacrificing my last piece to push the ending int B-N endgame. My opponent said I should resign as he had a book endgame. I said, "Yes, but I don't think you can do it." Fifty move rule got him: B-N is tight and he was out of practice.