As with most questions posted on the forum we often go off on a tangent...sorry!
I also think that things may be getting a little too technical for you so here's a beginners version...
A decent battery charger
has the ability to charge in three stages; bulk when the battery
needs most charging
amps, absorption for when the battery is almost full and float for when the battery just needs to maintain a full charge. The charger
knows which one and feeds the necessary amps to the battery accordingly. Therefore, the general rule
is to leave it on all the time. A battery only has a limited number of cycles so depleting it and recharging it will eventually kill it. Leaving the charger on all the time prevents the cycling. If you power up a 12v device while the charger is plugged in, rather than draining the battery, the charger kicks up a notch and provides the extra power for the device and that's what probably happens when your hear the charger's fan kick in.
So, a basic answer to your question is; 'yes' I would keep my charger on at all times.