If your folding rig can power them and it can take the heat then get them folding away if you want.
One thing to remember though, folding puts more strain on the GPU than gaming, so they will get hotter than under normal use.
If you want a system that has a better watt to points ratio then you could sell the 590s and get a couple of 660Tis to go in your folding rig. They draw less power and so will run cooler. You will be down a bit on PPD but the system will be more economical.