From one of the bibles in the 1999-present era:
"...poll respondents are chosen purely at random, from all geographic areas of the United States except the Los Angeles area. At the beginning of each question, the MC will announce how many answers on 'on the board.' This explains how many answers will be considered correct, regardless of how many answers were given to the question. In all cases, only the most popular answers will be displayed, and all answers receiving the same number of responses will be displayed if any one of them is displayed."
The latter bit is clarified by an attached question, which also confirms they poll 100 people, and not a larger sample to make a 100% percentage. The question "Name something people keep in their glove compartment" is set to be played with the top 6 answers on the board. The 7th through 9th answers got 2 responses each, but the show chose not to use them. Per their own rule -- a rule at the time mind you, it may no longer exist -- they'd have to use all of those answers, or none of them, because they had the same number of respondents. The remaining 1-point answers are there as well -- TIRE GAUGE, NOTEPAD, MACE and (shockingly, IMO anyway) SUNGLASSES.
-Jason