I've never heard of a regulation, but its been that way for the last 15 years.

In the mech units I've been around, when the CDR has a fighting vehicle (tank, IFV, etc) and a HMMWV, the HMMWV is -6, and the fighting vehicle is -66.

Also since I've been in the Army, we've had freq-hopping, encrypted radios, and used Hollywood callsigns almost exclusively. Every commander I've ever been around has used the unit call sign with the -6 expander. (Dragon 6, AllAmerican 6, Devil 6, Warrior 6, Thunder 6, etc). This is true from company level on up. Inside a platoon, the PL is the company call sign, then 16 (first PLT PL), -26, -36, etc. PSGs are -5 or -7, 1SGs/CSMs are usually -9 (or sometimes -7), XOs are -5. Primary staff are usually -1, -2, -3 and -4, but then they get garbled (because CDRs, XOs, DCOs, 1SGs/CSMs use their numbers) and usually have some two digit expander.