Most slot machines, Cindy, offer an incentive to play the maximum coin amount. For example, two-coin inserted may pay 2,000, but three coins pays out 5,000, a bonus of 1,000 additional coins. If this were the case with the machine you had in mind, then I would always recommend playing max coins.
But if the machine is a straight multiplier, say for instance the top jackpot line pays 1,000 with one coin inserted; the second pays 2,000, and the third 3,000, then, yes, occasionally betting less wouldn’t make a difference.