Everything depends on variety, zone, health of the tree, stage of development, weather this month, etc. etc. Pat answers are worth what one pays for them.
Chappy, more specific questions about specific trees you have might be more easily answered! Check out my site, www.bonsaikc.com, and my blog, Sashi-no-eda Bonsai Blog, for some good information on a variety of subjects.
Brent Walston, who is quite knowledgeable on developing stock for bonsai, wrote a bit about trunk chopping and growing large trunks here. Some discussion on trunk chopping can be found here. Personally, I have trunk chopped in the early spring with good results. This has given time for buds and then new shoots to develop and harden before the onslaught of Michigan winters.
It seems to me like most trees do well when chopped in the late winter/early spring with the exception of maples which do best if chopped in mid summer. (I don't really work with tropicals so I don't know about them)
What an outstanding article and piece of material Chris. It seems like the last pictures were taken back in 2005 however. Can you please post some new pics of this tree or send them to me via email? I as you know adore Tridents and I would love to follow the progress of this tree.
If you notice, I mentioned that the tree is no longer among the living. Ambitious root work combined with traveling full time and a lawn guy who moved the sprinkler conspired to deprive me of a tree that was coming along very nicely. Mostly I blame my mother.