To test trees properly for comparison with one another, you should do it:
1. During the sap flow season,
2. All about the same time, at least for trees you're considering keeping/thinning in one group.
Tree sap sugar content will change within a few hours, day-to-day, within a season, and from year-to-year. You either need to check them all about the same time, or test them repeatedly to get an idea of how good their sap sugar is over several years. Realistically though, for thinning, you don't ever need to compare a tree in one bush versus trees in a totally different area.