Most experienced high-power fliers recommend using a new 9V battery to power the altimeter and fire the ejection charges for each flight.  Why?

After I use a 9V battery in a rocket, I remove and replace it with a new one for the next flight.  I toss the used battery into a box of batteries I have for use in remote controls and other things around the house. When I use the batteries in some other device, they work fine and seem to have almost a full charge.  So, why don't I resuse them in my rockets?

The quick answer is "why risk an expensive rocket just to save the dollar or two cost of a battery?"

A better answer is that, although the majority of the time the battery is fine after the flight, there's a chance that it may have discharged more than normal or was damaged in the flight. 

The high G forces of a flight and recovery could damage the inside of the battery.  Unlike AA, AAA, C, and D "batteries" (which are actually single cells), a 9V battery is made up of multiple cells.

A Look Inside Typical 9V Batteries

The connections between the cells and terminals of a 9V battery may be damaged by the rocket's flight or landing. Testing with a voltmeter might not reveal the problem.

Even though it's not likely that the battery will be damaged by the flight ... why risk an expensive rocket just to save the dollar or two cost of a battery?