Newbie painter here. Is it okay to throw rags and paper towels that soaked mineral spirits into a plastic bag and then into the trash can?
No it is not safe. The plastic bag will most likely make it worse. I have seen just an open pile of rags (not bagged) smoldering in a jobsite waste can at the end of a work day. I removed them and set the pile in the yard just to be safe, it was a pile of ashes in the morning.