Spoilers should be added in pairs. I noticed a significant difference in high-speed handling and stability when I added my whale tail (previous owner had installed a chin spoiler but no rear wing). If you never exceed 70 mph, you probably won't notice a difference, though.
Check out this link
http://www.pelicanparts.com/techarticles/911_speed_prep/914_speed_prep.htm for a detailed discussion. Search for "STAGE III" on the page to find the section where the info about spoilers begins.