What situations are commonly accepted to be ok for someone to wear full medals vs. ribbons on their Army Dress Uniform?

To wear full medals is the highest level of formal dress, particularity if you don't have "Mess Dress"

I think it's more the expense than anything else that prevents people from wearing full sized medals. For example, I went on Medals of America and built my medals as if I were going to wear them. I was lazy and skipped some devices, but the effect is the same. The cost? $345.

Not too many people wan't to plunk down several hundred dollars for something they'll wear every couple years, and is no good the instant you get another award.

If wearing full or mini medals do no include the ribbons like OSR, ASR. Just leave em out