Re-heat, I have no doubts as to the usefulness and safety of beta-blockers, and your point about low heart rate is taken. I doubt that there exists any research evidence in the area of safety in low heart rate, but at the end of the day, the use of beta blockers when the heart is beating slow, is contra-indicated.
Weighing up the risks and benefits, my opinion would be avoid, if our heartbeat is low,(and 46 is low, albeight due to fitness), particularly if there is an alternative.