As the wise man said, "Opinions are like arseholes - everybody has one. However, unlike arseholes, you should get yours out every once in a while and give them a thorough examination under a strong light".
Simplistically, you can always supply a lower-drawing load from a higher-capacity supply. This is because the high capacity is merely a measure of how much current it can supply before the voltage starts to droop. If a supply is rated at 2.5A, that doesn't mean that it forces 2.5A into the load; merely that it can supply it if the load wants. In the case of your phone, it doesn't want to draw more than 1A, so that's all the supply will give it.
Now, before the smart folk leap in with a 'yes-but', yes, it is possible with a really (really!) poorly designed power supply that under-loading it will cause it to go unstable, but even with the poorest of poor, the load has to be in the order of under 10% of rated (ie in this case, under 250mA) before it would start playing up. If you're using such a supply, your phone is going to die from the high transients that the crappy supply lets straight through when your motor starts or stops, so instability is the least of your problems.
What to do? Start by purchasing a charger from Apple (they're actually made by Belkin) and make sure that you don't buy one on super special from a dubious source. Then, if you really want to be sure, unplug the phone/iPad while you start and stop the motor. If you're lucky, your 12V outlet will be switched on your avionics switch, and you always turn it off when starting or stopping the motor (don't you? ).