I usually wait until the tree is dormant (leaves all off, and been chilled for awhile). I think it'd still work if you do it now (in theory there's more energy still in the stems than there will be later), at least if you're planning to root the cutting soon after you take it. Not sure if there's any real downside to taking cuttings early, other than that you're taking a little more energy away from the tree. However, the wood hasn't hardened off as much yet as it will have done once it reaches dormancy, so it might be a little more susceptible to infections or something if you were to store the cutting for later rooting. I've rooted cuttings in spring, summer, fall, and winter, so I know it can be done. Informally it looks to me as though cuttings taken any time can root if you root them right away, but if you have a gap in between when the cutting is taken and when you try to root it, then cuttings taken while dormant work best. (That's about storing the dormant cutting and it remaining viable over the storage period, rather than about viability right away). Whereas green wood roots OK but is more susceptible to problems. Not a scientific answer, but those are my thoughts/observations.
Mike