Based on my experience, there is not a only and true answer to this question. You need to base in your experience because depending of the problem you are solving, you will need to activate some flags or other. What in a programs gives you good results in other can be the opposite. Some profiles and similar tools and your experience will tell you what to do in each ocasion.
Keep in mind that if you have to do always the same... so, the compiler will do for you without ask. It depends on compiler as well.