I think it is best not to touch them. I find that most CJ Sponsors usually offers a variety of banners and banners sizes which you can use.
I have resized cj/advertiser banners before. Sometimes they look okay, other times they do not show very well. Just make sure you keep the perspective, for instance, if you make a 468 width x 60 height banner 1/2 the size, go with 234 width x 30 height -- I've also made them 1/3 the size. I simply divide 3 into the width & height, and subtract that number from the original width & height. For instance, 468 x 60 would be 468/3= 156: 468-156=312 (new width) 60/3= 20; 60-20=40 (new height) Now banner would be 312 width X 40 height. If you don't stay within the perspective, it can get really funny looking.
I have been placing 468x60 cj ads in my posts over the last couple day just as a trial and I have been resizing them to 400x51 due to the width of my blog. I have no idea if this is against TOS but it what just what I needed to do to improve to overall look of things.