Using a different radio will not cause you to roam any more or less than you otherwise would. The "radio" that you flash is simply a piece of software that controls
how your phone communicates with cell towers, not
which towers it uses.
Most providers have their own radios (software), and each one has certain set of instructions regarding how to modulate the CDMA signal, how much power to use while doing so, and how to handle your data connection. Switching between radios will allow you to find one that works the best for you.
Rest assured, though, it won't all of a sudden cause you to get massive roaming charges on your bill.
