When should I take a curse in response to Torturer?
Short answer

You should take a curse when discarding two cards would hurt your strategic plan, but you should also take into account your curse total compared to the other players and how it's likely to change.

Long answer

I 'm not going to dwell on "obvious" circumstances like


There are no curses left
Your hand is all victory cards
etc.


Under normal circumstances you should make the decision based on strategic (long-term) factors. If discarding would mean not being able to buy a card you need in the long term (this usually means 5+ cost cards, and of course not all of them) then you should consider taking the hit so that you can advance your plan.

If all you were going to buy anyway is a "nice to have" card, then taking a curse is probably not warranted.

In a 3- or 4- player game,


you should gravitate a little more towards discarding in the beginning of the game, as taking curses too early (before you have the chance to load up on necessities) might leave you too far behind the other players
you should consider the possibility of multiple Torturers being played before your next turn; in this case, taking one curse and then playing a good turn might be warranted because with multiple players playing Torturers you can never be sure just when you 'll get the opportunity to play your turn unhindered