I didn't think twice about having one (it was such an offhand comment from the doctor) until the dentist got all balky.
t half-assed layperson lecturepants
The word "murmur" can make dentists twitchy, mostly because it can be both a diagnosis in and of itself and a symptom of something bigger and worse. Usually it's just a diagnosis - innocent murmur, functional murmur, pulmonary flow murmur are all diagnoses that just mean something in your heart is just a teeny tiny bit funky, not enough to cause any problems ever but just enough that your heartbeat sounds just a little different. Apparently in some people there's a little extra noise that sounds like a dove cooing or a musical note, or sometimes even a little click. It's a totally meaningless normal variant, like lefthandedness or widow's peaks or whatnot.
Sometimes it's a symptom of some Big Bad in the heart -- a hole where it doesn't belong or the lack of a hole where it does; these are louder and angrier and not pretty-sounding at all (I had a BF once with mitral valve prolapse, and when I rested my head on his chest I could hear a sort of slurping sound in between the BA and the dum of his heartbeat), and they usually mean surgery or a cath procedure or meds or whatnot (including antibiotics for dental work, since the human mouth is a giant germ pit and if the foul things in your mouth manage to crawl inside a bleeding gum and ride the bloodstream to your heart, bad shit can ensue).
And there are lots of people, usually freaked-out parents, who will hear their kids' complicated diagnoses and just shut down out of fear and walk away with the word "murmur," so dentists and other professional folks tend to get twitchy when they hear the word. If they don't know your full history, they have no way of knowing whether you're saying "murmur" because you know you have a slightly warbly but normal heart, or you're saying "murmur" because there's something really wrong and you've blocked out or just don't know the ugly details.
t /hall