Got tired of constantly getting leaks in my map, so I surrounded the whole thing with nodraw to contain them
I actualy still do this while the map is in alpha state (realy, if this extra processor cost would already be a problem in alpha, there are much bigger problems in the map). So for timesaving i actualy think that during alpha its a good thing to do that. Although when doing that a normal compile on vvis should still be done since the small tricks a fast compile can normaly do, these no longer work with such poor skybox. It shouldnt be any problem to render almost the entire skybox around it because in the later versions of the map, the detail is going to be way heavier.
But with that, i still try to seal as much as possible (generaly the side skybox textures that go all the way to the top). This helps later on when going for a beta since you already should have thought out most of the skybox areas. So you arent going to misjudge positioning as fast. The remaining minor gaps (usualy below the skybox level) can usualy be solved in beta where you will have to alter at least half of the brushwork for detailing. And when its only these lower gaps, even with such huge box skybox around your map, the effect is going to be minimal.
There is no need to perfectly seal an alpha, you want to be able to quickly change the map. Any effort you spend into perfectly sealing is going to be a waste of time the moment you need to edit it.