It's all about timing. I'd hate for a newbie to read this and think they can mess with the roots any time of year. Early spring is really the safest time of year to do root work....after that could be a death sentence.
Ryan, You may be right, but then you might need to expand the statement to "It's all about timing AND climate."
Up here in the Pacific Northwest, "Bonsai Heaven", where almost nothing ever dies, it's a much different climate than yours. I've had no problems at all doing fairly extensive root work on boxes any old time of year, except I don't recall ever doing any in mid-winter.
Even mid-summer, the recovery from emergency re-pots with moderate root pruning (after a dog knocked several off their perches) once impressed me quite greatly with phenomenal root growth, some of the most impressive I've ever seen on any species.
About a dozen years ago I got a sickly large one in the sale bin at a Memphis nursery in mid-summer, and that very evening I chopped it extensively, root pruned extensively, put it in a bonsai pot, and put it out in the sweltering Memphis heat - recovered immediately.
Three years ago in summer I bought 2 small ones and 3 slightly larger ones in nursery pots for a group planting, and transferred them immediately onto a shallow slab after significant root pruning - none of the five showed any set back at all.
Two years ago on a hot sumer day, I got a 2" trunk boxwood from a sickly looking hedge, dug it up, and immediately transferred it to a fairly shallow bonsai pot that required moderate root pruning to fit in - it never missed a beat, growth taking off almost immediately.
This year, based on such past experiences with them, I took a huge, severely root-bound nursery box during an unusually hot spell in late spring, and unceremoniously removed about 2/3 of the roots (and all the soil with fairly aggressive root hook work), while transferring it to a training pot - it never missed a beat, and roots were filling the pot in no time.
Of all the trees I've worked with, they seem the most oblivious to even drastic root work, and the most phenomenal at quickly filling a pot with new roots. I might add, though, that all my boxes are Korean ones - I find them the most attractive - and perhaps other species don't fare as well.
Of course, neither our summers nor winters here in the Pacific Northwest are severe, nothing to speak of at all (except in terms of precipitation), but as I said my oldest box did fine with rough root handling in the midst of Memphis summer heat. Santa Cruz, where FourMileMarc lives, seems a fairly benign climate, but I could be wrong.
G52