I disagree with the pressure gauge needle reading being the sole determinant of whether the FE is still good. Where I live, there is a fire code that requires all fire extinguishers in commercial buildings to be serviced every two years and recharged every 7 years (or something like that). The reason for this is that the white powder inside the FE can consolidate to form a big "clump" which won't come out of the nozzle if and when the day comes when you need that fire extinguisher to work properly. In that case, the propellant gas will come out of the FE, but not the white powder.
So, every couple of years, I have to take my three fire extinguishers in for "servicing", and basically, all that involves is putting the fire extinguisher in an apparatus that holds it horizontal and locates it's center of gravity. The fire extinguisher is then shaken and the location of the center of gravity determined again. If the white powder inside it has clumped together, then the center of gravity of the FE won't move because the powder inside didn't get redistributed when the FE was shaken. If it did move, then it's because the powder was redistributed when the FE was shaken. That determines whether or not the powder inside it is still loose and will flow freely when the FE is used, or if it's clumped together so that it won't flow out of the FE as a thick "fog" of particles.
And, every 7 years I have to have all three fire extinguishers recharged, and that basically involves replacing the powder and propellant in the FE and costs considerably more than just having it serviced.
If you're required to have your fire extinguisher recharged, THAT's when you want to pull the pin on it and use it LIKE you would in an emergency because the powder and propellant is going to be replaced anyhow. But, be prepared for a big white clowd of dust to come out of it that will sit in your driveway for days. (Maybe go to an empty field to use the FE.)
There are different fire code requirements for fire extinguishers. For example, here in Winnipeg they made it mandatory that all fire extinguishers in commercial buildings dispense their powder through a hose, and not just a nozzle. Apparantly, people were saying that holding the fire extinguisher so that the nozzle was pointed at the base of the fire was harder than just holding the fire extinguisher and pointing the end of the hose at the base of the fire. So, whether or not your fire extinguisher needs to be equipped with a hose or not depends on your local fire code.
Also, fire extinguishers are rated according to the weight of the powder inside them. My local fire code requires that I have a five pound fire extinguisher on each floor. That means there's 5 pounds of powder inside the FE. But, other places could require a 10 pound FE instead.