Some of it is situational though. I only sometimes say hello. I also worked as a janitor before and have no problem with people not saying hello. The bigger issue is if they are ignoring you and messing up your work (getting in the way, ignoring you when you need to get their trashcan from their desk, being an excessive slob like I'm here to clean but you don't have to make my job harder just because you don't have to clean it up yourself).