Assuming you refer to the RR method: Usually - with an unknown patient - you should measure on both arms, left and right.

In Brazil we measure the blood presure in the left arm. The values obtained in left arm are a little higher than te right ones because the arteries branchs at the right side. But this diference is not statistical different. If patient is mastectomized or removed the linphatic ganglions you cant measure in that arm. At the first evaluation both sides must be verified. Check the statement from circulation journal.

http://circ.ahajournals.org/content/111/5/697.short