What's the best way to straighten them?

Have you ever had any that were so bad you really couldn't make them work?

i would pin them to something - cork board or styrofoam or anyway you can.....spitz with water and let dry. Might put a bit of steam to it as well.

If they are printed off-grain - forget it. I won't use them. Now I check fabrics over carefully before buying panels.

If they are printed on-grain, but the fabric has gotten skewed in processing, I will try to "train" the fabric -

first by machine washing and drying - sometimes that does the trick

if that didn't do it, then I will resort to the old-fashioned method of pulling on the fabric to make it "go the right way" - and then will try to press it down with starch or sizing - much as I prefer not to use starch or sizing

If they are really wonky, would a wonky border/sashing around it work? Kind of like a crazy patch style.

I haven't had any that were way off, just a little and I've put extra wide sashing around them and squared them all the same.

If they are printed off grain, I would not buy them.

My MIL once bought some beautiful cat panels that looked like watercolor paintings. They were on sale, so she bought eight of them. She went to look at something else when they were being cut, and she got home to discover half of them were pulled/printed off grain! She still has them, I thought maybe they could be cut apart for broderie perse-style applique...I hate the thought of wasting them.

I would try to "true" them with starch. Sharon Schamber has some good videos on how to do this. Here is a link to one on how to true a block (would apply to a panel too; just need more space to pin):
http://www.youtube.com/watch?v=v-3RIWhBvcA

The trueing keeps everything lined up while you are piecing and quilting. Something that starts out wonky benefits from more quilting rather than less, because the quilting will help keep the fabric from going wonky again.

I can't speak to straightening up panels, but I've had to do that with several pieces of Boy Scout fabric I've been working with recently. I used my old folding cardboard cutting board that I used for sewing patterns and pinned one selvage edge in place, then pinned the opposite side, making sure the pattern lines were straight. Then pinned the other two sides. In one case I started with dry fabric and sprayed it with water after pinning, the others were wet to begin with. Then I just let it dry in place. Time consuming, but it worked beautifully.

I won't buy them if they are off too much.

Try Crazy patch on the ones that were too bad off...