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It’s not a sin to play short-coin, but there’s a defined rule for doing it.
On most video poker games, as soon as you move down from max-coin (also called "full-coin"), you give up a percentage in the return. Why? Because you lose the bonus for the royal flush. Hence, if getting the highest rate of return is your goal, you have to play full-coin in most cases. But with a goal of playing at the lowest loss rate per hand, full-coin is often not optimal.
To see this, you have to calculate the expected loss per hand, which is done by multiplying the casino advantage x the coin denomination x the number of coins played. So, for playing 5 coins on a 9/6 machine, the equation is -.005 x .25 x 5 = -.00625 —- a loss of a little more than half a penny per hand. Doing the same calculation for short-coin play requires changing both the number of coins and the casino edge, which increases, due to the loss of the royal flush bonus, to -.019 (98.1% return). This calculation yields loss-per-hand numbers of approximately 2¢ when playing four coins, 1.5¢ for three coins, 1¢ for two coins, and .5¢ for playing just one coin. Notice that at one coin, the loss per hand drops below that of 5-coin play, making it the superior option if limiting losses is the paramount objective.
That’s the answer to your question. In video poker, when playing a 5-coin game, you should play either 1 coin or 5, and under no circumstances should you play anything in between.
For more on the effects of short-coin vs. full-coin play, read Bob Dancer's new book, Video Poker for the Intelligent Beginner.