what ever ph you have. its better to have a stable ph then to try to mess with it. swinging ph will harm them more then a ph lower or higher then what you think they should be in.
the pH coming out of your tap is the pH you should have in your tank. don't mess about with your water chemistry or you'll create an instability which will result in a multitude of problems.
Also, if you go to a LFS, there water will most likely have the same ph as your's, so they will already be used to the ph. As long as you acclimate the fish properly, and keep the ph stable, the guppies should be fine.