Of course members of all four houses can have (or not have) their own sense or code of integrity, but I think it's rarely a defining trait in Slytherins or Ravenclaws. With the examples you mentioned, Slughorn and Snape, I wouldn't say integrity is at the core of who they are, compared to say, Cedric, Ernie or Neville. The kind of integrity I mean is the basis for pretty much all your decisions, and it's imperative in your choices. Cedric couldn't take the cup because he felt Harry had earned it just as much, Ernie couldn't stand by having questioned Harry when it turned out he was wrong, Neville could only join Voledmort if hell froze over. Slughorn took a lot of convincing to tell Harry what he needed to know because his sense of self-preservation and pride was stronger, and even then it might have been the felix felicis more than Slughorn's need to do the right thing. Snape is a bit more difficult, but I'd argue that his duty towards Lily was motivated more by his own guilt and a desire to justify his actions towards himself than wanting to do right by anyone else.

So in a way I think my notion of integrity is more rigid and defining than what I perceive as the Slytherin notion. It's not an optional thing in some areas of my life, it's at the very center of who I am. Not saying Slytherins or Ravenclaws don't have integrity, just that it's not a defining trait. The same way members of every house can be brave, but it's a core trait for Gryffindor.