Actually if you shoot raw its not good to go below base iso. Recently i learned that when you go under base iso the camera just overexposes the image and pulls it down in software to give you longer exposure times. With raw you can just raise shadows in post, its easier than pulling lights down. Negative side effect is highlight blowing out easy when going below base iso. So if you shoot raw then just stay at base iso and raise shadows in post. And if you really need the longer exposure invest in a ND filter.
It depends on whether or not you have a lot of highlights. As you said it just overexposes the image, which you should do if you can get away with the highlights.
