This discussion has been going on for while and a recent ruling by a Nevada judge has stated that casinos are responsible for the tax on comped meals that are provided to gamblers. The main logic behind the ruling is that if you are paying with comp points, you are paying with a different form of currency but you are still paying for the meal. The main discussion focuses on points. It implies that casinos will not have to pay tax on promotions where you are offered 2 free buffets when booking a room of for earning a certain number of points as long as points are not the method of payment.
This difference does play into the comp game a little. If you go to a buffet and pay with points right now, there is no tax. If the buffet price is $11.99, you pay $11.99 in comp points. If you charge that to your room, the total charged to the room will include tax (which I think would be a total of $12.96, but I cannot promise that). If you pay your room bill with comp points, you will be paying that tax (even if you are paying the tax with comp points). That is why whenever I am given the option to pay for a meal with comp points, I do it.
Here is a link to the VegasINC article about it.