The logic proffered above is germane. If you don't have a number, I would not call myself a PE. Wait until the state issued number is assigned to you. Your business cards and other correspondence can have PE after your name if your address is in the state that you are registed in. So, if I am registered in Iowa, my firm is in Iowa, I can put PE after my name, on my business cards and in my signature block as long as my address shows that my office is in Iowa. Now, if I go to Idaho for a project, and am assigned to the Idaho office for the project, I CANNOT put PE after my name IF I use the IDAHO office address. Why? Because I am only licensed in Iowa! If I apply and receive reciprocity in Idaho, allowing me to become licensed in Idaho, then I can use the PE title after my name while on the Idaho project. Why? Because now I am licensed in BOTH Iowa and Idaho!
Be careful here-- you can use PE after your name as long as the address of your correspondence is the location of where you are registed. If you are not registered in that state, you cannot use the PE title.