We are currently in the middle of a storm which may produce extensive tree breakage. Since I'm starting a salvage nursery, I'm thinking that broken branches would produce lots of cutting material alongside being paid to cleanup the wreckage. However, it is the wrong time of year to take cuttings; to late for hardwood, to early for softwood or semi-hardwood. This scenario will go on all year, due to storms or tree trimming. Should I make attempts to propagate cuttings regardless of the "proper" stage or not?
Also, are ornamental crab apples generally grafted to rootstock, or not?
From my reading and limited experience, no question about cuttings can be answered without reference to specific tree species, and even then answers are variable due to the wide-ranging diversity of tree genetics. Trees range all the way from "sometimes a skilled nurseryman with an automated misting tank can get one cutting in 300 to take root under lab conditions" to "I cut a six-foot chunk for firewood in January and then in March I drove it into the ground small-end-first and tethered my goat to it, but now in August it is a twelve-foot tall tree." In other words, the dreaded permaculture "it depends".
That said, when my trees take damage I often try to root the cuttings. My dogs ate a branch off my Wolf River apple tree last summer, and (in high summer) I turned that into four cuttings that I stuck in a pot. This spring, one of those cuttings has leaves, although given how hard apples are to root, I still suspect it's a last-gasp effort stealing nutrients from the cutting wood and probably doesn't have roots. On the other hand I cut a bunch of elderberry cuttings from a fruiting stand last summer that was about to be sprayed by the power crew, and they not only took root right away and flowered in my pots before fall, they all seem to have survived the winter.
Thus my real answer to your question is that the "proper" time to take cuttings is just a rule of thumb. It's the time when the consensus of experience agrees your cuttings are most likely to take root. So when the time is not "proper" but you have the cutting materials, what's the downside? Your time and propagation space, that's all. If you have enough of those, I see no reason not to do it.