Very important for NiCd batteries, since they develop (as it's called) 'memory'. Without discharging/recharging, you'll only get a flight or two out of a NiCd rx battery. You'll double that with proper cycling.

I don't believe that NiMh and Lithium-ion batteries have the same issues, so if you are considering additional packs and don't want to worry about cycling, you might look at those types.

You should always keep a battery tester handy that can load-test. That's the only way to be sure of how much you got left on a given pack. Without a load, you might read 5.5 volts, with a load you might be at 4.8 (which is too low to start a flight on). I've seen this first hand.

Any discharger will do - I've seen some that are simply a string of light bulbs connected to battery plug. I use a combination charger/discharger unit myself.

Cheers!

MrJB