No they don't need to be. Better if they aren't as it lets a little air in.

The purpose of the dome is to maintain the humidity at a reasonable level, as long as that is accomplished, don't worry about air leaks. Al

I leave mine slightly askew so they are slightly open to prevent too much moisture from building up. It's always a trade off between enough moisture and enough air circulation. This works for me.

My "dome" is covered air tight and after a week, I open it about one inch for one or two days than I seal it again. I have had 85 percent success with this method.