It's not necessary unless you're using an ultra wide angle lens and causing severe vignetting due to the stacked filters. Otherwise, it's really up to you, but just note that stacking filters tends to increase flaring, and some might say, a loss of image quality.
The very first time I used the CPL, it was placed on top of a UV filter. Then I experienced vignetting when i shot at 18mm of my 18-200mm lens. Therefore, it is better to use the CPL alone without other filter.