Turn Raises -- How do you handle them?

[O Doyle Rules]

How do you respond to turn raises in either a heads up situations or with two opponents? Specifically, if you are holding top pair/top kicker or an over pair to the board after the flop.

If the board is non threatening, do you always assume a set or two pair and go into a call/check/call mode? Do you ever re-raise with top pair/top kicker or an overpair heads up?

If board is threatening, do you ever lay it down to a turn raise? Heads up? With 2 opponents?

Does anyone always fold to a turn raise in these situations, when it is heads up and the pot is small? And if so, do you feel it is +EV?

For sake of arguement, lets assume this is online with no particular read on your opponent.

Thanks to all who respond.