I have lost many cuttings in cups due to either over or under watering. I try to be very careful about giving too much water, but I have drowned a few. I started being too careful about watering too much and killed a few with dehydration. I learned that if I kept the humidity over 90%, I really never needed to water the cups at all, until they were ready to go into soil. That's the reason I have found high humidity helpful; It lets me avoid the whole issue of watering while in cups.
I have
this mini greenhouse set up by the window in my office. I put my humidity meter inside and it was staying around 55%. I stuck a vaporizer (the kind that makes steam when you have a cold) in it and turn it on a couple times a day. It maintains one blast of humidity for several hours. Since I'm always sitting next to it, I keep a close eye on it. Otherwise, I might install a humidistat switch.
The big problem I've had is getting the babies out of here. It can be tricky adjusting the plant to drier air and brighter sunlight without killing it (or at least it is for me). I will finish out the plants in these cups, but I've just started going from rooting in coir directly to 1 gallon pots of Pro-Mix outdoors and misting several times a day. They look good so far.
Now that I have my humidity tent worked out just the way I want it, I now plan to stop using one at all.