There's no advantage in holding on to frequent-flyer miles, as there's no way of knowing whether they'll be worth anything down the road. Miles typically expire within 18 months (though that deadline may be deferred if you keep your account active, either by flying or using an airline-reward credit card that is linked to your account), so get a free flight — or golf clubs, electronics, magazines or anything else the airline lets you trade miles for — while you can.
If you're still not sure whether to buy a ticket with miles or cash, here's a quick equation to help: the value of a frequent-flyer mile is about 1.2 cents, according to an analysis by Consumer Reports, and most airlines charge at least 25,000 miles for a round-trip flight within the U.S. That adds up to $300 at the 1.2-cent rate, so figure that you should pay with cash if your flight costs less than $300 and with miles if it costs more.