In last weeks Dresden Files game, the players followed the bad guy using a tracking spell, and found him in the projects at an apartment that they knew little about. Seeing his car, they decided to set it aflame to get him out of the apartment (and to identify it).
The wizard's spell was a bit too ambitious, and ended up turning it into a roman candle (how I love compels), but the actual plan was successful, as the bad guy came running out to see about his car, and the wizard was barely able to get out of sight before he exited the apartment. As the bad guy was dealing with his vehicle, the group snuck into the vacated apartment. They found that there was still someone there, lounging on the sofa, obviously having partaken of some 'herbal entertainment'. As they entered, I had them make an alertness roll, ostensibly to see something in the apartment- but it was actually an empathy roll, to determine if they saw through the person's deceit. He was high, but was still quite lucid, and was the actual threat.
My question is, how valid is deceiving the players about what they are rolling for in order to maintain a layer of the unknown in the game?
(Just for completeness, I'll state that the players are still unaware that the person that they fought was just a messenger/dealer/mercenary and that the doped up stoner was playing them...)
UPDATE: Though the question could apply to multiple circumstances, the accepted answer brings up a FATE specific point, so I changed the tags and update the question appropriately.