It's not actually the presence of metal in a microwave that's a problem.
After all, the inside of the oven (and most of the rest of it) is made of metal.
The problem is with sharp edges, and / or short gaps between pieces of metal which can act as spark gaps.
If you look at a microwave cookbook, it will show you how to put aluminum foil over the small ends of some meats (like fish) to keep it from cooking too fast or burning, etc.
Always follow your owner's manual's instructions (and don't throw away the manual!) and refer back to it first with any questions.
You can probably cook with the grille / rack in place or not. Again, check the manual. It may not address the issue. If not, then you can assume it's safe to leave it in.
It's also very important to keep a microwave clean. Grease and food debris can build up and cause arcing, which can damage a paint job in an instant.
This is especially important in models that have racks. Keep an eye out for grease buildup where the rack meets the mount, and where the rack mount meets the cavity wall.
Take care.
William E. Miller, AS-EET
prototech@usa.net MicrowaveDisplay.com
"Ask Mr. Microwave"