WildBill said:
It is proper strategy to call with middle pair most of the time in heads up situations. As I said before, raising and play agressive early, call late. Basic shorthanded strategy, really applies to about anytime you are going at it heads up from before the flop.
which is why i raise off the bat with big hands, you get callers. You make a decent sized bet on the flop, they call and the pot keeps growing, by the river the pot has grown to the point that you can make a pretty big bet and they have committed themselves.
calling with middle pair as a rule is a very risky proposition against players that realize that you will do that. Once in a while, if a player is getting overly aggressive it is good to call with middle pair just to slow him down. even if you lose the hand it will make him think twice before he tries to bluff you off a pot.
Like all poker, it is all situational.