I'm no expert, but I remember being told it should be done when the trees are taking nutrients back down to the roots in fall. If anything, my guess would be too early at this point. This popped up in a google search.
From the dark side, an article:
When Hack-and-Squirt is Effective
“This method is effective almost any season except early spring,” said Jim. “When you have that heavy sap flow and sap rise, it can wash the herbicide right out of the cut. Otherwise, late winter, summer and fall are fine. In the South, the sap starts flowing in March or into April. It will start later in the North.”