I don't think there's any hard and fast rule on glass thickness. As long as it lets through enough UV to expose the emulsion, you're good.
The one factor that may influence your glass thickness would be how big your exposure unit is. If you're making a blacklight bulb table that can expose two screens at once (resulting in a pretty massive size piece of glass), I'm not sure you're gonna do too well with 1/8".
So I guess it depends largely upon what you're about to build.