Not that my opinion means much on this front since I'm still just sort of easing in to messing around on Twitter, but unlike this site it's not quite the same function -- rather, if this is a home base, Twitter communities is like an outpost.

If one wants the outpost to increase in size and population, leaving it open would probably be the correct course of action.

By starting with it closed, however, you get to have a bit of a feel for the people who were here and joined so you can mentally connect one to the other and tell who is being genuine, and who are just trolling.