Having a little debate with my Mom about microwave turntables

[glatt]

Putting a plate or bowl slightly off center on a turntable will in theory heat it more evenly than if you center it perfectly on the turntable. Every microwave has "hot spots" depending on how the microwaves bounce around withing the reflective walls of the oven. The waves are emitted from one source and create a pattern the way they bounce off the walls. Sometimes they are in phase and create a hot spot, sometimes they are out of phase and kind of cancel each other out. The very middle of a microwave may be a hot spot or a cool spot. Probably varies by oven.

I've seen physics experiments done in microwave ovens without turntables where a glass casserole dish is packed tight with marshmallows and the marshmallows are cooked under close observation. As soon as they start to melt, the oven is turned off. You can clearly see where the hot spots are because the surface of the marshmallows is melted in those spots and nowhere else, and you can even measure the wavelength of your oven by measuring the distance between those spots.

Having said this, there's probably no noticeable difference centering your food rather than putting it slightly to the side on the turntable.