It's not clear because they don't know. As far as I know, at its best it's a luxury and at its worst it could make it worse. I don't really recommend to use it, especially if you don't have easy access to it. You would be adding another potential source of problems for a benefit that it may not exist.
Quote from Vizzy's SN Bible about this:
"One of the symptoms you may get is a fast heart rate (tachycardia). It is completely normal for the SN process and does not cause any pain. Your heart will beat faster as if you have done some physical exercise or you are watching a scary film. The drug recommended is Propanolol. In the opinion of the author of this document, it is a luxury item and would not consider the effort in purchasing it to have any significant effect on you not being distressed.
It is a prescription only drug and just adds another complication to sourcing and potentially using if you are already on prescribed treatments.
A dose of 400mg of propranolol taken with the antiemetic (metoclopramide) was previously advised, finally some reports have suggested that the use of a beta blocker is a luxury that serves no function other than alleviating the tachycardia (racing heart). It is argued that the raised cardiac oxygen requirements will speed death (presumably from cardiac arrest). However, until more information is available, the benefit of using beta blocker cannot be established. The beta blocker has, therefore, been removed from the protocol."