Someone told me that she uses weights in her wall hangings and it makes sense but do you?

I haven't. It does sound like a good idea though, as long as it doesn't weigh down the seams.

I haven't and mine seem to hang ok.

I don't use weights in my wall hangings. I think that if you can't get it to hang flat, no amount of weight is going to fix it. What you can do instead on a small piece, is put triangles on all four back corners. You can then slip doweling under the pockets on all four sides if need. They work kind of like a stretcher frame if you trim them to exact lengths.

My suggestion too :thumbup:

Try the triangles like this: http://www.alzquilts.org/sleeve.html

you can always put a dowel at the bottom one to keep it flat.