In the Year of our LORD 2006 A.D. here in the Federal Republic of the UNITED STATES of AMERICA there have been a few Court Cases even in 2005 .. with the rulling the place that serve food are not owed any funds concerning tipping. If the person booking the function wants to tip they can or NOT. It is NOT REQUIRED.
There are also many cases where the IRS investigate "service industry workers", where tips are the norm. This is because it is understood that those service workers will make a certain % of their income via gratuties.
In fact there have been cases in the past where gratuity income was assumed, even when it wasn't declared.