I guess I look like a cheapskate here, but I don't tip for perks I am being given by the cruise line for my loyalty. I'll go to the Diamond lounge and have a drink and perhaps a snack and then I leave. I've never felt I needed to tip. To me it is no different than going to the Captain's welcome aboard party or something similar. Do people tip at those? I don't think so.
I do give fairly lavish cash tips to my favorite bartenders and waiters at the bars.