I'm sure you will get many different answers here. Personally, I like to get my sphagnum nice and fluffed up. I then put a layer in the bag and put my cuttings on top, I then put another layer on top. I use 1 gallon bags and I remove all the extra air without compressing the moss. I do this because I believe the moss holds plenty of air and then it keeps everything in place when I move the bags around. I flip my bags once or twice a week. It takes about 4-5 weeks for mine to root, but I'm not in hurry so that works for me. I open them a few times to check things out, starting around 2-2.5 weeks.
You keep them all until they get big enough that you can see which is for sure the best and keep the healthiest one for yourself. I don't keep back-ups, but I do give them to friends that I think they have a chance of living with; so I have a backup(for cuttings) near me should I need one. It is also good to keep some yearlings of different varieties around because they mail well. Then in the late summer or fall you can offer up your extra trees for trades.