In self defense you should keep both eyes open. Your dominant eye should override the other one and the sight will be superimposed on the threat. This takes practice and you will hate it at first because it does not seem precise. You need all of vision you can have on the threat. If you close your non-dominant eye you will only see the area around the front sight.
If you are just target shooting, and not practicing, go ahead and close the one eye. It will be more precise.