It all depends on my situation and the player available. Last year, I was ready to drop my sleepers after week one to pick up Brandon Jackson, because of the potential situation he was stepping into. His potential upgrade over waiting and seeing on a guy was worth more to me.
In a normal year, when you just have guys who have big games, I'll take a shot if I am not super confident on my bottom of bench guys, whether they are sleepers or not.
This year, I have already dropped one of my sleepers, Andre Roberts, when Daniel Thomas was dropped. I felt like he could offer me more over the long term for the season, given the receivers that I have. One of my other sleepers, Jermaine Gresham, I'm likely to hang on to a bit longer, because I only have Greg Olsen above him on my roster and neither guy is a proven commodity yet. Then again, I don't have any super deep sleepers this season, because my draft had a ton of late round value that I wouldn't really consider risky plays (Burleson, Bernard Scott, etc.)