Most frown upon stacking filters. Especially if you have lower priced ones that are not multicoated. Light can bounce between the filter elements and cause problems in your shot. Some people use a UV filter just as a lens protector. They don't actually provide that much effect to the shot. Also if you have a wide angle lens, if you stack filters you may get vignetting.
I have good multicoated filters but rarely stack them. If I want to use a CPOL thats all I mount. Some lenses I use a protector, some I don't.