Humidity is very low, can I use humidity domes on seedlings?

I have 2 seedlings about 6 days old. They are looking good as far as I can tell. I have them in a small grow box. The temp is 75-80. The problem I am having is the humidity is around 20-30%. I tried hanging wet towels. I tried a bucket of water. None seems to work. So can I place a ziplock baggie over each cup to raise the humidity? Or do I even need to worry about it?

You must be down around Hemet or the valley, but if you mist them for a few days, maybe a week, week and and half, they should be big enough to survive a low RH like that.
Keep trying the wet towel in front of your fan, tub of ice, seal the room if you can keep temps under control and the humidity will stay in the room.
I wouldn't worry about getting it up too much, unless you start seeing problems.

Low humidity isn't usually a problem at all, worst case scenario the media will dry out quicker and you'll have to water more frequently.