Queen: Not usually. The checkmate is fast and you have to think about stalemates. Rook: Yeah. Use it to stop the king moving sideways and force a retreat.
Should a Minor piece assist a Queen/Rook vs King endgame?
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#1
No. You should be able to checkmate KQ vs. K and KR vs. K without thinking.
Just make sure it is out of the way.
Should a Minor piece, like a knight or a bishop, help a Major piece during an endgame, and if it should, how should it help the Major piece?