From my experience there are two good times to perform a trunk chop on Zelkovas, elms and most other hardy deciduous trees. The earliest is just prior to or during spring bud swelling but before the leaves start to beak bud. The second is any time after the new spring growth has had a chance to hardened after the buds break into leaf.

A tree can be trunk chopped in the early winter after full dormancy with some success. However I find that the trunk may experience winter dieback further down the trunk than was planned resulting than a less than ideal design.

Depending on you local weather conditions and your ability to care for the tree, "now" might be a less than ideal time. If chopped now this late in the "normal" dormant cycle the procedure may stimulate new growth during less than ideal weather and light conditions requiring protection from the elements than you are willing to provide for. This practice is based on my climate and the time I am willing to devote to my trees.. This time of year I am doing everything possible to keep my trees dormant for as long as possible so that I do not have to jump through hoops to protect them from late freezing temperatures and frosts.

After the spring bud swell and prior to leaf break is a good time to trunk chop. However you must be able to read the condition of the buds and be willing to protect the tree from that last hard frost of the year. If the new growth is damaged for whatever reason before it has the ability to replenish new energy stores the trees ability to recover is greatly reduced.

I have had the majority of problems trunk chopping deciduous trees just after leaf break. I assume that the tree has put most of its growth reserves in new leaf production i.e. branch tips. When chopped the leaf production area is removed robbing the tree of its growth energy resulting in the trees being severely weakened.

I have had the best results trunk chopping during the time period starting after the first flush of new leaves have hardened and the end of summer dormancy. I have trunk chopped elm trees as late a mid July with very good results.

As a general rule new growth has hardened off when it changes from its emergent growth color and darkened somewhat. I have two rules of thumb if I am in doubt of the trees condition. If the new growth can be pinched with my fingernails it is still tender and wait. If there are more than 7 leaves on the major branches the growth is past the tender stage regardless and can be chopped with little risk.

This is what works for me and my growing conditions. Your climate or method of caring for trees may yield different results. I hope my ramblings provides some ideas.