Yes, a service guy will ensure they will be working, when needed, but - some of that you can do yourself.
Your post is a great reminder for those of use that don't think of safety and fires.
It takes more then simply rotating them end for end - a guy should use a rubber mallet and hit the bottoms when rotated, to break loose the powder and mix them up.
This should be a maintinance chore every month. Then you would only need the service guy to check them about every 4yrs or so. Tempature and climate play a role in fire extinguisher maintinance, how the power settles and packs.
Moving machinery, it can really pack in, so, shake them bottles up to keep them in condition.
Of course, checking / tracing power cables at least yearly, for wear is perventive maintinance and can save them from fire . .  . . .
I know, I know - how many guys do that ? Well, if they have time to polish and shine, they have time to trace hoses and cables for wear - At least I make that a priority over shining, in the winter months. It saves on fires and hydraulic blows/leaks, when the tractor is needed most.

Edit: I should learn from my own advice - this spring, I had a big hydraulic supply line spring a leak, from a rub mark. It wasn't much of a rub mark, but enough to spring a pin hole and spray high pressure oil all over the side of the tractor - which could have caused a fire.
So, I'll be doing a better job replacing rubbed hoses / cables