Most manufacturers provide a built-in diode to prevent reverse discharge. Also, modern trickle chargers have microprocessors that take care of issues like overcharging, short circuits, reverse polarity, etc.
Hence, if you buy the latest trickle charger kit, you can leave it plugged in all the time. Brands specify if they do not have this function. Regardless, most new companies will provide features that enable you to leave a trickle charger ON all the time.