The first issue is dormancy.

This usually occurs earlier in colder climates. Here, in San Diego, it usually comes toward the end of the year. In the 2009-2010 season some varieties were not dormant at the end of January, and a few never really became fully dormant, because we had a very mild winter. Cuttings that are fully dormant store better and longer with less loss of vigor and viability. I had some cuttings last year that were still a bit greenish, and shipping them within a week or two of making the cutting left them quite viable, but after a month they were getting limp, indicating that they were too old and not usable.

The second issue is freshness.

While dormant cuttings can be stored, often up to a year (I am still starting a few of mine from February), the fresher they are, the better chances for success.

The third issue is temperature.

In the eastern United States, where winters involve frost and freezing weather, and often very cold temps, people do extraordinary things to protect their trees from those temperatures. They store them in warmer out buildings, wrap them with insulating materials, or even bury them. This is done to prevent exposure to cold temperatures, which can damage or kill a tree, or portions of it. If portions of it are killed, then it obviously won't releaf in the spring and bear fruit. But it also won't be viable as a source of cuttings to be used for rooting. In that climatem if you want to be sure that you will have cuttings in the Spring, you take them in the fall before they are exposed to damaging temps, and thus not usable.

Finally, if you take them in the Fall, and have the facilities (greenhouse or equivalent) you can start plants during the winter, when you might have more time than when you are busy starting a garden in the Spring.

I am experimenting with starting some plants now, from cuttings that are NOT dormant, but fairly hardened (not green) to see if they will root (I am sure they will) and if they will take more readily (I think they will).