As Eastbay1K says whether I tip or not depends upon what the voucher says. Some HGI vouchers say gratuity included, some gratuity not included. If gratuity is not included I tip based upon the level of service, usually $2 or more. The server usually brings me coffee, water, and refills, and in some locations will deliver the omelet I have ordered (for those HGI's that have an omelet station).
See also:
http://www.flyertalk.com/forum/hilto...ver-tip-3.html