I honestly don't know. I've worked almost every area of nursing, including CCU/ICU.
If it were critically important that the BP be "that" accurate to within a few points, the patient would have an arterial line and or a Swan Ganz line. (I may be showing my age? Do they still use Swan Ganz lines?)
I suppose in an ideal controlled situation the right or left arm could make a few points difference. I have seen much bigger problems with inaccurate BP's with nurses using cuff's too big or too small for the patient or the patient lying on their left or right side.
Many things can affect the BP. Repeatedly taking it over and over in a short period of time on either arm will make a difference. The patient being stressed or in pain. So much can affect a BP. I just can't get excited over the right or left arm issue.
Left arm is more accurate as it is closer to the heart.
No, it's more accurate for the left arm. Right arm is more accurate for the right arm. This is related to the old reason why we wear wedding bands on the fourth finger of the left hand: in olden years it was supposed to have the most direct connection to the heart. Not. How this got carried over into a rationale for taking left arm BPs I will never know.
There are times when you would always, always check both (aortic malformations, aneurysms among them). Always check both if the radial pulses are much different.
What many people fail to understand (or maybe they never learned) is the BP is not a static measurement. Your BP varies widely over the course of your day, from minute to minute, with breathing, venous return (more venous return, as with exercise, makes your cardiac contractility faster and stronger -> increased BP), heart rate, endorphins, and many, many other things. Your BP of 132/80 this morning at 10:10am is not the same as it is at 10:30 am, or 2:00 pm, or 7:45 pm.
BP measurements are therefore just data points, not independent status indicators UNLESS COMPARED to something. Obviously, an adult with a BP of 60/20 (in either arm, intraarterial monitor, popliteal...) is in some trouble. However, an infant with a systolic pressure of 60, not so much. An adult with a blood pressure of 120/80 may be dangerously hypotensive compared to her normal, and you would be unwise to say, "Gee, nice numbers" without knowing what it has been before.
Therefore, if you are monitoring serial BPs on someone to check the progress of therapy or the decline in an emergency, always use the same arm (or leg-- yes, you can take leg BPs too) so you are always comparing apples to apples. Always chart what arm (or leg) you used.