1. a nice mid-level vintage road bike that's been rebuilt can be worth 5-6 times its frameset only value (or even more).
2. a visable dent in a full build will drastically cut the whole value, not merly a fraction of the frame. if selling, it's often better to part out the build.
3. it's always better for the marketing of the bike if the ding/dent/bulge is inconspicuous.
4. yes, i ride one. it's a tiny bulge underneath the downtube from a racing bike that was crashed in a hay bail and later straightened by the builder. you can feel the bulge, but it's difficult to see and can't be felt while riding.
5. if the dent were major and conspicuous, i would pursue fixing it if the whole build were worth it.