I would like to know people's opinion on what I believe to be good manners in checking down with another player when somebody else is all in. Ie somebody all in with 2 callers.

I have had people [badly] bluff me out of these pots and then end up doubling the short stack because I at take their betting as a "I'll handle this guy" nudge, but I find it rude. Is it an ethical play to bet into a side pot with somebody all in?

I can hold the nuts and will check it down. If somebody starts betting at me, then they're doubling me up/paying me off handsomely, as far as I'm concerned, but if they are happy to check down and eliminate a player with me then I will take that with the lost profits I could make.

Am I being too nice? Should I be taking the opportunity to increase my stack? Would it be considered different in SNG games from ring?

I imagine that a tournament situation would give this type of play more influence on the game but I would also consider it ruder than a ring game when there is no aim of a first place finish.