All my muffin recipes state "bake on insulated baking sheet..." or "fill muffin cups, then place the muffin pan on an insulated baking sheet...". Why do I do this?
Well, often homemade muffins become toasty on the bottom. Depending on your oven, the bottom can overcook before the rest of the muffin is properly baked. If you don't have an insulated baking sheet, that's no problem. Put your pan in the upper third of the oven and bake a minute or 2 longer than my instructions state. Test muffins and record the correct time for Your Muffins in Your Oven!
My recipes are all baked in the center of my oven, on an insulated sheet. My oven is an old Wedgewood Range from my husband's grandmother: (pic to be addd!) It bakes just slightly more hot at the rear, so I often fill the back muffin cups a tiny bit more.