If you take hardwood cuttings that have not put out leaves yet, autumn, winter or early spring, generally the cutting won't open more leaves than it can supply water for. Cuttings taken autumn, winter and early spring before bud break are referred to as ''hardwood cuttings''. Many trees root reasonably well from hardwood cuttings. For example, blueberries, cutting are taken in autumn, after leaves have fallen. Over the winter some callus forms. Spring only a few leaves will open, roots actually form sometime in late summer early autumn. They are not handled or moved until the following spring because they do not form roots for over 6 months.
If your cuttings already have leaves open, then it is appropriate to either cut a few leaves off, or back. Cutting large leaves in half is common practice. Usually cuttings taken in spring with leaves just opening are the least likely to survive, because the water demand to open and mature leaves is the greatest. These are called ''softwood cuttings''. Certain perennial herbs root best from softwood cuttings. Majority of shrubs and trees do not have high success with softwood cuttings.
Cuttings with leaves are usually taken after the summer solstice, and are referred to as ''semi-hardwood'' cuttings. This is after new growth has slowed or stopped for the season. The stem has lignified to some degree, will feel woody, rather than soft and flexible. Leaves are all fully expanded and have been mature for several weeks or months. Majority of trees and shrubs that can be propagated by cuttings will be successful as semi-hardwood or hardwood cuttings.
Cuttings of woody trees and shrubs are very species specific as to what it takes to get them to root. Dir's Manual of Propagation of Woody Trees and Shrubs is the bible for methods that would yield commercially viable numbers of rooted cuttings. Mist beds, with heat mats to keep media warm are common recommendations. Hormone treatments, at various concentrations are often recommended. Sometimes it is noted in Dir that using no hormone worked better than the addition of a rooting hormone. In general I find using hormones hit and miss, and hormones in excess of ''directions'' can often inhibit roots. Too much is not a good thing with rooting hormones.
But keep in mind, the trees and shrubs don't read the books. It is possible to have low levels of success, like maybe one in ten, or one in 25 attempts, with getting cuttings to root. At the hobby level, having only one or two out of 25 cuttings root might be all the hobby grower needs. So there is no reason to not try to root cuttings out of season, or in less than ideal conditions. The old fashioned just stick the twig in a glass of water has a surprising success rate. It is very far from 100%, but works more often than one might think when reading the propagation literature.
So give cuttings a try, any method, any time will likely give you ''some chance'' of success. Never say never. Just give it a try. Right now is when most bonsai people are pruning a lot of trees, no reason not to try to strike cuttings. Even if Dir says it is not the right time.
