On the one hand, it's your money and you can do anything you want to with it, so from that point of view it's covered. Extra money for extra service I should think perfectly permissible, value for value received, as long as it really is considered extra - that's the gratuitous part of "gratuity," after all.
But in the real world of present-day America it isn't usually gratuitous at all. Wait staff are paid less well than they otherwise might be because of that. "Wages, tips, and other compensation" is what goes on the old 1040. I think in an idealized Randian world wait staff are properly compensated for their jobs per se and tips really are gratuitous. In that sense the current system is corrupt, at least from Rand's point of view. Value for value received, no cheating, and where a gratuity is no longer gratuitous and the wait staff suffer otherwise, it's cheating.
Not much evidence to back that speculation up, however. It isn't apparent that Dagny tipped Akston at his diner for the best "hamburger sandwich" she'd ever eaten. In chapter 10: She rose, paid her bill, and said, "Thank you, Dr. Akston." In fact, she even accepted one of those magic cigarettes with the dollar sign on it from him.
Dang skinflint, drug-addicted capitalist... ;-)
I’d argue, though, the pay of wait staff is much closer to free market principles than other wages.
Most restaurants don’t have problems finding wait staff. If restaurant wages were too low, no one would work for them.
I’ve been surprised at how much a good waitress can make a decent restaurant. And, the nice thing about cash tips is being able to keep a few bucks without the IRS knowing about it.