Definatly not an urban myth. Dry-firing is one of the worst things you can do to a bow. It puts excessive stress on the string and cable(s), cams, axles, limbs, riser, everything. When theres not an arrow on the string that unabsorbed energy causes the limbs to overextend. This action pulls on the limb bolts and pockets and on the string and cables causing cracked or shattered limbs, warped cams and axles and damaged strings. In rare cases, it has caused the cams to actually break, shearing the string posts off.

All in all, take every precaution against dryfiring. The easiest is to just nock an arrow every time you are going to draw the bow. Remember to always point the bow in a safe direction. Another precaution is to purchase a "no-fire" release. Basically this is just a release with a hook instead of the normal jaws and has no trigger. We use these at the shop I work at, and they are priceless when letting customers look at bows and test drawlengths and peep locations. And the last way to prevent it is to always keep your finger behind the trigger unless you intend to shoot.

Hope this has cleared it up for you backyardbowhunter.