If all you can find are pre-trimmed green beans, I recommend getting frozen instead (see previous question). So far, I have been disappointed 100% of the time with pre-trimmed beans; they are almost always slimy, no matter how recently the package swears they've been prepped, and the ends turn brown when I cook them, which means I have to do my own trimming anyway. Unlike butternut squash and carrots, which I'm happy to buy pre-cut, I find that green beans are one of those foods you're better off prepping yourself.