Since wireless plans these day pretty much all have free long distance and phone number portability, and since you can manage your entire account online, the actual address where your phone is registered is irrelevant. However, you probably do want a friend or someone you know at the registered address, just so they can receive any physical mail regarding your account and pass it onto you (or at least not send it back as undeliverable).
According to the data I have, the lowest taxes are in OR, NV, ID, DE, MA, MT, and WV in that order. Or, a better tactic might be just to go to MyWireless.org and check the taxes in the states where you have friends who's mailing address you can use.
I haven't actually tried this, so I can't vouch for the effectiveness or even legality of it, but I love the idea conceptually. I'd love to hear from you if you or anyone you know does this. Happy tax-reduction!