Um, party-pooper alert.
No veterinarian can tell you a cat doesn't have a heart problem based on x-ray and auscultation (listening with a stethoscope). It cannot be done. Cats can have heart disease that does not cause a murmur and does not show up on x-rays. If anyone tells you different, they are sadly misinformed.
I very much wish that us worrier types could have a vet listen to our cats, pat us on the back, and say "don't worry." Some of you may know that a vet did this to me years ago, with a stray I planned on keeping (the only stray I've ever found and said, he is meant to be with me) ... and that cat died in fulminate heart failure shortly thereafter. This doesnt' mean that every cat who gets a clean bill of health is destined for disaster, of course ! But it does mean that that reassurance is false. Cats with no heart murmurs can certainly have hypertrophic cardiomyopathy (HCM):
http://www.ncbi.nlm.nih.gov/pubmed/19480619
On the other hand, cats can have murmurs that are benign. (Dog murmurs indicate disease the vast majority of time, in contrast.) Not all cats with murmurs actually have heart disease, and the only way to tell the difference is with an echocardiogram. One study with not many participants (so it may not be representative) showed only 53% of cats with murmurs had disease:
http://www.ncbi.nlm.nih.gov/pubmed/21276739
A study done in Europe showed a much higher number:
http://www.ncbi.nlm.nih.gov/pubmed/21276739
(In my experience, it is not that high of a percentage, FWIW--we see a lot of benign murmurs.)
Panting is not normal for cats. Panting can occur in normal cats when overheated, stressed, or with overexertion. (Those African cats are panting because it is very hot out there!) Normal exertion (getting the zoomies around the house, roughhousing with another cat) should not cause panting. I agree that a lot of young cats will play too hard, as Cindy says, and end up having to catch their breath because they've overexerted themselves and gotten too hot.
The first symptom for many cats with cardiac disease is congestive heart failure, aortic thromboembolism (ATE), or sudden death, so personally I would screen any cat who gave me any reason to think that there might possibly be an issue. A few cats will show "exercise intolerance" and we've seen appointments for that reason; it seems to be particularly people who have more than one cat and notice that one cat poops out before the other.
If I had a kitten who panted when chasing the laser light or feather thingy and I did not have my job (working as a cardiology nurse for animals), I might wait on it esp. if there was no murmur and no other sign of disease and the kitten was able to play hard on her own without a problem. If I had a young kitten with a murmur, I would get an echocardiogram no matter where I worked.
In a young cat, if money was a problem, I'd spend it on a cardiologist doing an echocardiogram rather than a regular vet doing an EKG and BP and some kind of ultrasound.
Sorry to spoil the reassurances in this thread ... I think that everyone should have the most information and make their own decisions.