I currently use the Phase 5 on my 308 AR Service rfle. It is allowed.
Rule 4.1 on the AR Service rifle allows it, pg 40:
An extended bolt release is permitted on both right and left-handed receivers, provided there are no permanent alterations to upper or lower receivers.i) Relieved (Milled/Cut) lowers are allowed, but the use of an extended bolt release that requires a relief in the receiver is not permitted. Extended bolt releases must be attached on the original manufactured side of the lower receiver.ii) Billeted uppers and/or lower receivers are permitted.iii) Ambidextrous safeties, ambidextrous magazine releases and ambidextrous charging handles are permitted, provided no permanent alteration to the receiver was made to install them.