We've actually engraved some tricky conchas - in the flat - then domed them without noticeable distortion. Almost all domed bracelets we work on are engraved flat and then domed.
Depends on how much you stretch the metal, in which direction(s), and most importantly - whether or not you are using a conforming type of die. If the work bottoms out, you'll definitely damage it.
Using one of the thick urethanes to press into has never caused a problem... yet?
Brian P. Marshall
Stockton Jewelry Arts School
Stockton, CA USA
209-477-0550
instructor@jewelryartschool.com