As others have replied, I've left most of my chargers plugged into an outlet for years without a problem.
They don't continuously put out any voltage/amperage all the time, well unless you always have some device plugged into them. They only provide power when in use and otherwise just sit there in an idle state. Using a Kill-a-Watt, a common power meter, just checking three different chargers -- one is using 0.01 watts and the other two are using 0.00 watts when idle. (... with the 0.01 watts adapter probably being because it has a continuous red/white LED indicator light, the other two are just your typical wall-wort adapters that get packaged with any phone).