I generally do not tell player TNs or rolls of opponents. I try not to tell any layer any DC, TN or Difficulty Level from any game. A lot of games publish DCs and TNs in the players rules, and that's cool if they know them, but if I can, I like to keep them behind the screen.
Heck I would love to see a dice system that let the player roll, without them being able to see the result, so that they would not know what they got, only how well they did based on descriptive narration.
At the same time I try not to let them over-spend on Drama Dice. In fact I have been known to ask a player "Are you happy with that?" when I know they are close to getting a TN and need to spend a DD to get them over the hump.
As for Henchmen and Villain characters I try to do two things: a) decide if I will spend on a roll BEFORE I roll and either roll or don't roll before I learn from the player; and b) I will usually spend on a roll if the character comes within 5 of a TN (sort of like how "the house" always has to bet on 16 or lower in Black Jack).
In the case of an Active Defense , when I already KNOW the TN, I stick close to those rules as guides.
In most cases the group will out DD me, so I am cool with spending one to help keep the enemy in the game, so long as it is an interesting character and situation.
Side note.
One thing I do in my, mainly just D20 games, is to have the players make a couple dozen D20 roles on a piece of paper for me. Then when things come up that I don't want the player to know...say like making a Perception check to notice a ambush...I can use their roll to narrate the event. There's nothing like saying--"okay you guys are going along the woods and....make a perception roll..."--to kill the mood.
The sheet of dice rolls is also good for Finding Traps when the player should wonder if "You don't find any traps" means that there are no traps or they just didn't have a good enough roll to find some. If they know their roll, they can tell, guess, which might be the case.
Something like this is a bit harder to do in 7th Sea, where there's a plethora of dice combos and adjustments.
