I tend to like the IM approach for when I have a question or suggestion on a work. If it is posted as a wip then I will put the comments in the comments. Just because one posts a wip and requests suggestions does not make the final work any less the artists. It is always up to the artist to evaluate such feedback (be it technical or aesthetique) and shape the work in his or her own fashion.

Pass no temptation lightly by, for one never knows when it may pass again!