From what I understand, casino hosts are not allowed to accept money but will accept gifts. I sent mine 2 bottles of wine from a local winery after a great trip to the Rio last September. My host went out of his way to make sure my stay was enjoyable so I thought he deserved a little something.
I don't think a gift is necessary or even expected but it surely doesn't hurt anything. I don't think the quality of your gift will have any effect on the comps for your next trip.
I've read on some posts that people never see their hosts and don't know until they check out if anything will be comped. Mine stops by a few times throughout my stay just to say "HI" and to see if there is anything I need. This is the kind of host who deserves a gift.