For us in the PNW, anytime is okay if you can put up with their looking ratty for a while but now is good since they're going to look ratty anyway.
I like to do my dividing in June/July, right after the first set of roots and leaves have grown, particularly if I'm doing micro division. Gives the cutting more time to recover and settle down before the fall/winter. If you have mature clumps and you simply wish to take divisions that would still have upwards of 6 or 7 shoots -some people call them eyes- you will still be fine right now, let's face it, there will be plenty of water around to help establish the roots in their new location. Be prepared for them to come back up next spring looking smaller than what they are now but don't worry, the year after they'll be pretty much back to where they are now in terms of leaf size and height. When all's said and done, Hostas are quite resilient. Just make sure that if you're planting them under trees I'm told that you don't bother with maples, apparently they'll choke out the Hosta eventually.
I like to separate mine when the leaves are in the "bullet" stage. In the spring/early summer, the first thing you see coming up are those pointed tightly curled leaves. When they get about two to three inches high, I'll take a sharp shovel and cut the rootball in sections to be planted elsewhere, that way, there's no wilting or leaves to rip, the new leaves unfurl just fine like the plant (s) was never even moved.
