For most restaurants you can cancel anytime - even on the day of the reservation. If you had to give a credit card number at the time you made the reservation (e.g. dinner shows, CRT) then you have a deadline for canceling or you will be charged a no show fee or the full amount of the meal, depending on the restaurant.
I couldn't tell if you already have a reservation at Les Chefs, but are thinking of canceling it and taking a chance on walking up or if you have a reservation elsewhere that you're thinking of canceling in favor of Les Chefs. If you know you want to eat at Les Chefs I would make (or keep) the reservation. It may be showing availability now, but they might be fully booked the day you're there and possibly not taking walk ups.