I would stick to collecting in spring or fall, not post-flush. Here’s my reasoning:
At bud break the tree is in an energy positive, if you collect it at that time it can regulate the amount of foliage produced to what the roots (that you’ve collected) can handle. Also temperatures are lower which lessens the amount of water needed for cooling (which is >95% of all water uptake). And it gives the tree more time to grow new roots before the growing season is over (from you nickname I take it that your from Norway which means you have a short growing season to start with so maximizing the amount of time for root growth is even more important). An added bonus is that you only mess with one part of the tree, the roots and you don’t have to touch the foliage.
If, on the other hand, you collect during the summer after the first flush has hardened you shock the tree more. The tree has already put out the amount of foliage that can be supported by the whole root mass and now all of a sudden a lot of it is cut off. On top of that it might be hot and the tree would need a lot of water for cooling and only has a compromised small root system to do that. If you reduce the foliage to reduce the water demand you’ve now disturbed not only the roots but the foliage as well.
Early fall is another good time to collect, especially some species, like spruce, as they experience rapid root growth during the vascular growth season (which is the latter part of the summer and fall). If you collect after the risk of heat spells the tree again needs less water for cooling and can concentrate all the energy it has accumulated during the summer to grow new roots before winter hits. This might result in less buds and a weaker push the following spring but that’s just the tree regulating its growth to what the root system can support.