If you have a four wheel robot, I would put the 40 (harder) tires on the front, and the 30 (softer) on the back.
When you're pushing, you want more traction on the back tires, and you want the front tires to spin rather than stall.
One thing that is really essential with these kinds of tires - you have to clean them, constantly. I use rubbing alcohol (70%), and a piece of paper towel. I wipe them as often as I can, and at least between sets, although sometimes there isn't enough time between individual matches.