Preheating the oven is not necessary to make your bacon come out well. I personally don’t preheat the oven because I just want to get it in the oven to start the cooking process.  By not preheating the oven, the bacon fat renders slowly and can result in evenly cooked bacon. But without preheating your oven you may have to adjust your cooking time a little so keep an eye on your bacon so that it doesn’t burn.