I'm not sure if there's any kind of valid technical specifics for what I do, but for the peace of mind it's worth it. I usually end up eating the extra shipping costs, but again for me it's worth it for that peace.
I like to try and use boxes that are larger than the standard. (I also use the ones with the two piece styrofoam insert rather than the panels) My theory is that temperatures take longer to move if there's more air mass in there, whether it's cooler or warmer. And the extra room for cushioning I like as well. I want to give them every chance I can, to get there safely and with the least amount of stress possible. I'm not sure how many exactly I've shipped, but it's been quite a few and I've never had any issues.
For anyone who is interested, these guys have very nice boxes:
http://www.uline.com/BL_2157/Insulated-Shipping-Kits
I really like the 8x6x7 for small animals, and the 9.25x9.25x7 for anything larger.