Having read a lot of fanfiction, I just realized that the Potion Master's hair has never been described in a flattering manner. The phrase greasy-haired has been used to describe Severus Snape in most fanfiction where Snape is portrayed as an antagonist.
Does this common theme have an origin in canon?
The following are a few examples of such fanfic instances for your perusal.
"I shall take your wisdom into consideration," Severus said neutrally.
"Also, try a different brand of hair shampoo."
A wry grin crossed Severus's face, and Harry thought it might have been, for the first time, that man's true smile. "Drop dead, Potter."
-Harry Potter and the Methods of Rationality