If the pot odds are good then it's a good idea, if they arent, than it isnt. If you are on a flush draw after the flop the odds are decent that you will make your flush, in that case I would do it.

However, if you dont even have a pair after the flop I would consider it a true "cold call" to continue to call hoping that you get something on the turn or river.

Personally I don't consider it a type of slow play. In my opinion, slow play means that you have a hand and you are changing gears to get someone to think you dont...not really a bluff, but more of a short term variance.