There are steps they take to reduce it, such as alternate legs and sleeves to accommodate gloves and boots, etc. etc. I guess if you consider the sheer amount there could be there is very little. Some games they don't even try to make boots work with pants.
But there is still a lot, especially with hair, for every race. If you want to have a variety of glamours you need to compromise on what Hairstyle you use, or live with a certain amount of it. So when people say they aren't giving Viera and Hrothgar hats because of clipping issues, it makes me wonder what exactly their criteria is for acceptable clipping. They obviously don't have a zero tolerance.