In searching, the internet seems very divided on this question. I want to achieve microwaving nirvana.
There are resonate standing waves inside the microwave. A microwave works at 2.4 GHz. The wave length of 2.4 GHz is 4.92 inches or just under 12.6 centimeters. This is a sinusoidal wave, so half of that wavelength distance is in the trough and half is in the hump of the waveform. These high frequency photons are bouncing around in a Faraday cage made of metal. Their pattern inside the metal box is fixed. The magnetron is off to one side and emitting the radio light from a fix position. As the waves of light bounce around inside, they tend to align into standing wave patterns. Some of the waves cancel out while others work together to amplify little extra energetic spots.
If part of your food is effectively stationary, like at the turntable center of rotation, there is a chance that a dead spot in the radio light wave pattern will form in that region and will not transfer energy to the food. The more the food is offset, the more it should cross points of radio light. So it is always more effective to offset the dish as much as possible.
If the bowl you are heating is smaller than the radius of the microwave’s glass-platter, then no problem, you can offset it to touch the outside of the platter and miss the centrepoint.
If the bowl is larger than the radius, I usually heat for half the time with one side of the bowl touching the outside of the platter, then slide the bowl to touch the opposite side of the platter, and then heat for the other half the cook time.
Even heat dispersion generally.
Fun experiment: take your turntable out, place in a large plate with pieces of the shittiest American cheese slices. We are talking kraft single serves. Splay them out, no deeper than one layer thick is needed, but make a full cover of the microwave bed (on top of the plate though. You will make a mess if not). Then you simply cook the shit out of it. You will see the cheese bubble and burn in the microwave wave peaks and you will see cold cheese in the wave troughs.
And then you pull out a ruler and measure how far apart the middles of the bubbles are. Look up the frequency of your microwave and then use that to calculate the speed of light with nothing but fucking cheese, a microwave and a ruler.
The turntable is there because the microwaves bouncing around inside the unit don’t do so completely evenly; there will be spots that don’t get heated as much. Moving the food around inside the unit makes it so one of those cold spots isn’t consistently in one place in the food.
So with that as a context, if a cold spot is at or near the center of the turntable, it’s not going to get moved around in the food when it rotates. So better to put the food away from the center.
My guess is that the edge or center is relative to the design of individual units, but after starting using the edge it seems to be more even so I’ve kept with it because the level of effort is basically zero.
Doing lower power for longer has far more impact on even heating if you aren’t making something that needs max power like popcorn.
This varies by microwave as not all microwaves perform the same way, but in my case, for more even heating there are benefits to placing the food just off-center. When the food is placed in the center it doesn’t heat as evenly, which is to do with standing waves forming inside the microwave, which leads to hot and cold spots.
