Discourteous, Unethical or OK

[KenProspero]

You're dealt rags in the BB, it's checked to you.
The flop doesn't help, either does the turn but you check as does everyone else in the hand.

After the river, it's either impossible for you to have the winning hand, or nearly so.

You're first to act. Can you just muck your hand?

Does the answer change if SB had called pre-flop and you're second to act?