The time to slowplay

From everything I've read, slowplaying is often a bad idea because it lets people complete their draws for free.

So... Is slowplaying always a bad idea when there's a draw of any kind on the board, and your hand doesn't beat the completed draw? For example if you flop trips/set and there's a flush draw on the board, then slowplay is a bad move, compared to flopping a boat when there's a flush draw, then slowplaying is a good play?

What if the flop comes rainbow, and you flop a set? Does it matter if it's low/middle/top set or not? Should you slowplay a set on a rainbow flop?

So do you only slowplay monster hands that aren't in danger of being drawn out? I could see this in a big multi-way pot, but in a HU pot, isn't taking the slimmer chance of him drawing you out worth it for those cases when he picks up somewhat of a hand on the turn/river?

Generally, I will avoid slowplay as number of players and number of draws increase. If there are draws that make a 2nd best hand to mine, I am more likely to slowplay and hope they catch.