I know this is really late but hopefully it will help others.
When you get your phone flashed you will more then likely have to be on a stock rom. This is from experience as I am a Page Plus dealer. Many roms do not support either the codes, or the drivers to allow your phones modem to connect with a pc to allow flashing.
Once your flashed to the carrier, you can use whatever rom you want. Keep in mind though that depending on what rom you use, they may access the raidos diffrently and your 3g data may or may not work. You may have to use a stock rom of try several roms before you find a reliable one.
Just a heads up in case someone didnt know, before you switch carriers and get all pissed with support.....aka someone like me.
If I am wrong about this please correct me and teach me a new way because I would really like to be able to flash more cm9 phones with out reverting them to stock first. ( just to save time. ) So far the major culprit are motorola phones.