My BP is a spaz and doesn't sit still. Both my boas and my JCP will sit on my neck/shoulders and ride around. When I took Rosey my 6.5' boa to the vet, she wouldn't sit still unless she was on me. The vet ended up just letting her sit on me while she examined her as her sitting on me kept her quiet lol. The vet and the nurse both were very amused how my big boa kept trying to get back to me whenever they removed her from me. It was proof, snakes do know their owners lol.
If your snake is tensing up on you, it means they feel insecure like they are going to fall or are startled. I personally wouldn't carry such a small snake around my neck like that. She can easily slip off you.
Me personally, I have never really viewed BPs as a social snake. They usually easily startled and very timid and shy. My boas, retic and dumerils are all way more outgoing. They constantly check stuff out and are always reaching out to get on to something. Rosey even had everyone at the vet office loving her lol. All the staff, doc and even the other patients parents all wanted to look at her lol. Most have never seen a 6'+ boa lol. And Rosey was mellow to all of them. She never even so much as pulled away or hissed.
Anyways, sorry for rambling but I wouldn't carry your snake around like that until she is big enough to completely hold onto you and even then, not so much with a BP.