Casino hosts generally toe the company line and (at least publicly) refuse cash gifts, although as you might imagine it's not like any host would truly decline the generosity. I suppose it also depends on the circumstance - I did regularly give one of my old hosts at Bellagio a red envelope every Chinese New Year.

With my current host, I try to take him to a nice dinner every so often, and bring him a bottle of scotch every other trip or so (although it's more annoying to do so now with the stupid TSA rules). Certainly you should not feel obligated to give gifts and there is no real expectation, but there are many ways to get around cash gifts if you choose to do so.