I have Walkers Low and the much taller upright N. subsessilis. I cut them both back after the first bloom flush as you did, and get a rebloom by early August. Then at the end of the growing season, I cut them back to the ground. I am always pleased with the fresh spring growth without all that 'brush' to clean up around it.

Catmint doesn't seem to care. There are things that have to be cut back in the fall, and things that shouldn't be cut back in the fall. It isn't in either of those categories. If I get everything that has to be done, finished, and get to it - great. If I don't, no harm done. I usually get about a third of them done.

Agree with mad gallica re: cut it back whenever you get to it