Generally, slowplaying is mostly not a good idea imo. It seems to me that these days people tend to play more tight then they used to so one of the main advantages of slowplay, to induce a bluff, is not really working anymore. Of course there are still opponents that would try to bluff you off but still it's usually better to go for value with your own good bets. Another thing is that it is very risky to slowplay average hands like TPTK, as you can be outdrawed very easily and it can all be very costly.
However, there are still few situations where it could be the best possible option. For example if you hit a really strong hand on a safe board like AA on the A93 rainbow or something, where it is quite unlikely that your opponent hit anything decent. It will be hard to get value from these situations so free card may actually help.
Another thing is if you're playing a real maniac. It's simple then. Just let him throw his chips away a slowplay all day long.