Welcome to the Modding community here at the V forum.
If you phone is already activated it will be activated when you flash/install a custom ROM.
If for some reason use should lose service (it has happened here and there), and you keep the backup of your stock ROM that you made in your custom recovery( this is called a nandroid backup), then you can restore the nandroid, activate, then restore the previous ROM's nandroid and everything will back just as it was before you had the problem.
For more info on getting the proper flash_image, recovery, and a selection of ROM's go here
All Things Root Guide