Always limping with JJ or TT seems pretty conservative. I don't know the math, but I'd guess that pre-flop, it will be the best hand most of the time. Of course, other players will almost certainly have overcards. The more of them who are able to see the flop cheaply, the more likely at least one of them will out-flop you.
Also, if you raise and then continuation bet, they might well put you on a better hand.