Hi Stephanie
I can only speak from a tbpi experience; I had my blood pressure taken several times recently in one day, and I discussed this subject with the nurse doing the tests. She tested both arms, then again about three hours later, and at all times the readings were pretty much the same for each arm. In theory, the bpi arm should show a slightly different reading, particularly if there is severe muscle wastage (which I have)but it didn't. We all came to the conclusion that taking blood pressure readings from either arm would probably be O.K. (I still would rather they use the 'good' arm, and always say so..)
Taking venal blood though, was a whole different ball game. I have had blood taken from both arms and on the same day, and by different people; sometimes it seems to be as easy to find a vein in either arm, but it is always (with me anyway) much harder to actually get anything out of the injured arm. Arterial blood was no problem from either arm, but again, I always ask them to use my left arm, even tho it bl*ddy hurts!!