Exactly. Play money is a great place to build bad habits. Even if you only put in the minimum amount and play low limit games like nickel/dime, you're still getting a better play experience, just for the simple fact that people have SOMETHING invested and will play more serious (though not much more seriously).
I don't know what the lowest limit is on Party Poker, but Paradise Poker has 1 cent/2 cent.
Quite honestly though, the real fun comes from having something invested in the game. It shouldn't be so much as to break the bank, but it should be enough to make it interesting.
