I think the only real problem with having no lid on the jar is that condensation will build up inside, then when you go to evaporate it the water will stick around and make your crystals "gooey" if you will. 

Don't stress out though, if you do get any water in your mix, just mix the crystals with warm to room temp naphtha, separate the water and the naphtha (They will not mix) and re-precipitate.  Actually, that would give you the opportunity to also do a quick sodium carbonate wash, and you'll have nice clean crystals when you are done