It is confusing, the advice about not fertilizing a flowering tree while in flower. There are several points to this. The main one is that it assumes that 'fertilizing' means high nitrogen, which maybe should be avoided for most flowering plants. But not all. Chojubai is a good example. This tree if anything flowers more when you fertilize it more, even when nitrogen is the highest number of the three macros.
But for most trees that flower before they leaf out, don't fertilize until after flowering. In general this will be a more established bonsai, and in general we wait a bit before fertilizing them anyhow.
And then, if you want fruit, then that's where too much nitrogen MIGHT be a problem. But not in all instances, Chojubai again being an exception, since the fruit will develop happily even with a lot of nitrogen present.
And then giving a tree some kind of fertilizer after flowering is a good idea since most kinds of flowering are weakening to a tree. After flowering a modest, balanced fertilizer with roughly equal parts of the macros in a 5-5-5 approximately, will be fine for any tree even if in fruit.
As for cuttings, just be sure it's rooted before fertilizing. And then when you do go for something mild, like a bit of fish emulsion. I think it's important to fertilize a cutting before the fall, because it needs to build up its resources having lost so much growing roots.