we just bought a new counter-top microwave to replace our old dead one & found that when in the exact same place, the new one is off-balance, rocks back'n'forth a bit. after discovering the little feet don't screw/unscrew, we fixed this by putting a little pad under one of them. we just want to know if this is okay or not. we're probably being overly paranoid, but am concerned the unit may be bent or something. it does seem to work just fine. there are no rattling noises. we are ok to live with the pad under the little foot as long as the unit can function normally for a long time, but if there's any valid concern about it, we'll take it back. i did call the LG customer service, but they were clueless. any advice is appreciated.
It's not really surprising that an example of the lightweight sheet metal that passes for an appliance these days is askew. I'd have done just what you did, assuming that there are no signs of damage or faulty assembly. As long as the turntable functions smoothly there's no concern with it being slightly out of level, if it still is.
The only concern I would have is if your little piece of paper under one or two feet was actually 8 inches tall; as the turntable inside might come loose form the roller tray and not turn.
Other than that there is no issue with the unit not being perfectly flat. Your oven has a magnetron that fires a high frequency radio wave pattern down a wave guide. Many units have a metal fan in the end of the wave guide called a stirrer that spins and scatters the waves inside the microwave oven cavity (where you place your food). The Radio Waves do not have an issue with not being level - you could actually turn the oven sideways, upside down and twisted and the unit would still work (as long as you could put your food at the appropriate relation to the wave guide). There should be a square plastic or cardboard like piece on the top inside of the oven cavity , that is the actual exit point of the wave guide - where the microwaves come out to blast your food.