Ooh this is a touchy subject. Personally, growing up my mom told me that EVERYTHING I did looked great. As I got older I realized what she was doing and now when I receive a compliment, I never know if it's actually truthful or "just being nice."
I think if the question is open-ended like "What do you think of my points?" or "Are my color choices okay?" it's different...obviously the quilter is looking for critique. But just to show a finished project and have someone start tearing it apart would be devastating. The quilter chose that pattern, fabrics, etc. for a reason. Unless they say it's not exactly what they had envisioned, I don't think it would be right to say anything about it.
As for the "If you can't say something nice, don't say anything" mantra, that's cool. But it makes me think...no one made a comment on my last BOM picture, does everyone hate it? :?