Keep in mind that the ammeter does not sense volts, it senses current, or amps, and is not voltage sensitive. Leaving it in circuit will have no adverse effects, and the fact of the matter is that a modern volt gauge will be more appropriate for telling you what's going on with your charging system than the ammeter. Most of the time your charging system is operating optimally the ammeter needle will be in the middle anyway, so no worries. Worst that will happen is the ammeter will peg to the charge side if your battery is REALLY drained, and in this instance you know something's wrong anyway!!
