I would personally wait until the buds are just swelling, to maximize the time frame in which the auxins will be produced. The auxins are what triggers root growth, naturally. Which is why we also repot this time of year for many trees.
Make sure your 6" pieces have quite a few buds, or else make them even a little longer. Clean up the end that will be rooted, with a very sharp knife. (like an exacto knife) You can taper cut, or you can make two taper cuts (one on each side). The main thing is to have a cleanly cut cambium layer, undamaged by shears.
I use perlite with the fines sifted and mixed with some Aussie gold seedling mix. Better than peat moss at retaining and evening the moisture. But we live in a desert here-- pure perlite can dry out in half a day sometimes. Wetter areas probably don't need the addition.
This is a goofy side note... but I have had a lot of luck wiring my cuttings quickly and giving them some movement just before trimming and inserting them. It only takes a minute, and if you are doing a limited number of cuttings it may be worth considering. My reasoning was that it is easier to handle them before they have a delicate new root system hanging off, and cutting off the wire later when they are rooted is less stressful than applying it. Especially with trees that get very stiff even when young... don't bend them too much, or you will damage the vascular system, but just give them some little movement. The twisted pomegranate seedlings that I treated this way are now rooted, (100% success!!) and will have movement from the base. The serissas are also very successful, although it isn't as crucial with serissas because they are pretty bendable. And anything that you may root for mame or shohin is good to get your movement into early. Just a silly idea....
Joanie