Generally speaking, I advise players to play max-coins when playing video poker. For most versions, this means 5 coins. The penny Player puts up 5 cents, the nickel player 25 cents, the quarter player puts up $1.25 and the dollar player has to put up $5 per hand. This is done for one simple purpose. On most video poker machines, the top payout - the Royal Flush - changes from 250 for 1 to 800 for 1 when that 5th coins is put in. If you are playing a Progressive, the only way to win that jackpot is to play 5 coins.
A payout of 800 for 1 on the Royal is worth approximately 2% of the total payback of the machine. A payout of only 250 reduces this down to about 0.65%. So, the Player is giving up more than 1.25% of payback if he plays below max-coin. In similar fashion, if the machine is offering a Progressive, which should push the Royal payout to above 800, then the Player would be surrendering even more payback by playing below the max-coin level.
The notion of playing max-coin does NOT mean you should wager 5 times the amount you feel comfortable wagering. Instead it means you should consider lowering your denomination to the next lower level and then play 5 coins. So, rather than playing 1 quarter, you should play 5 nickels. This, of course, assumes that all things are otherwise equal. It is certainly possible that when you go to a nickel machine (or change to the nickel option on a multi-denominational machine) that the paybacks may change as well and you may find that the payback on the nickel machine is well below that of the quarter. This makes things a bit more complicated. If the quarter machines pays 99.5% at max-coin, then it will be closer to 98% if you play 1 quarter. If the nickel machines pays 98.5% at max-coin, then you'll still be better off playing max-coin nickels.
There are a few times when you may want to play less than max-coin. The first is when you are first leaning how to play. As you are more apt to make mistakes at this point, you might be better off simply playing 1 nickel at a time. Yes, you will be playing at a lower payback, but at this point, your goal is to become a better player while playing on a real machine. Ideally, you'd spend most of your 'learning' time playing on your computer (or phone or tablet) at home for free ,but I realize that playing for free may be a lot less exciting than even playing for a single nickel.
Another reason that you may not want to play max-coin is your bankroll. If your bankroll is not large enough to support playing max-coin then you might be better off playing single-coin. Once your bankroll is gone, you're done and you need to make sure you have enough money available to ride out the cold streaks. Of course, one solution to this issue is again to simply drop down in denomination. So, this advice really only applies if machines of a lower denomination are not available. Since the advent of the multi-denominational machine, finding machines that play the denomination you want to play has become much easier, however. So, this second reason may have limited practical applications. But, if you find yourself in a situation where your bankroll will support 5 nickel play, but you only have quarter machines available, you may want to consider playing a single quarter as opposed to five quarters.
One critical point to consider. Just because you switch a machine from quarter play to nickel play, do NOT assume that the paytable is the same even if you are switching to the same variety of video poker. There are no requirements that state that a machine must use the same paytable when you move from one denomination to another. In similar fashion, don't assume that a bank of similar (or identical) looking machines all have the same paytable. Casinos frequently and presumably purposefully mix the machines up, making sure to sprinkle higher paying machines in with lower paying ones. I dare say that you may find no rhyme or reason to the pattern of machines on the casino floor.