Well, if you have them plugged into a strip, the strip has a 15A breaker, and the house has a 15A breaker. The zip-cord it's plugged into should be able to handle a bunch more than 15A (although technically it's a 10A cord, I believe...)
For a little extra, you could plug it in in the kitchen, which should have GFCI type plugs (assuming a modern house that is to code)
Mine stay plugged in, straight into the wall. [shrug] A lot riskier stuff plugged in than those! They're pretty tame.
You'd have to plug a LOT of chargers into a power strip to even pop its breaker (assuming no short circuits)
Unless you have a good reason to unplug them (dogs that like chewing through power cords, for example), it wouldn't (and doesn't) concern me much. Mine stay plugged in 24/7, whether they have
batteries on 'em or not.
Smouldering leaves and paper are a FAR larger risk, for certain.