The answer to this question can be found in chapter 10; however, the text doesn't explicitly tell readers how Ralph feels about Piggy's suggestion. Throughout the story, Ralph has been placing all of his hopes for rescue into a fire. He believes it is likely the one that thing will draw attention to the island to bring help. Interestingly, Ralph ends up being correct when Jack lights an enormous fire trying to kill Ralph. The main problem with Ralph's current fire is that it can't be seen as well as the previous fire's location.
“It was different when the fire was—”
The main goal of the current fire is smoke. Ralph hopes that it will produce enough smoke to be seen. That presents the problem of fueling the fire, and Ralph's group simply doesn't have the man power to tend the fire and search for fuel.
The fire was dying on them. “Two to mind the fire,” said Ralph, half to himself, “that’s twelve hours a day.”
“We can’t get any more wood, Ralph—”
“—not in the dark—”
“—not at night—”
Piggy's suggestion is to let the fire die down at night because the smoke can't be seen at night anyway. Ralph knows that Piggy is correct, and he reluctantly agrees to the suggestion. He isn't thrilled about not keeping his signal burning at night, but he also realizes that Piggy is right, and Ralph doesn't have a better solution.
Ralph stood up, feeling curiously defenseless with the darkness pressing in.