Once the temp drops below 160 or so it's a good idea to cover it - dust is covered in wild yeasts, which can cause problems for long term storage. At that point, DMS formation isn't really an issue, so covering won't cause any off-flavors.
The Fiesty(sic) Goat Brewery est. 2007 & Clusterfuggle Experimental Ales est. 2009
Planned: sLambic II, Flanders Red II, Orange Blossom Mead, Fat Man Porter
Primary:sLambic I, Janet's Brown
Secondary: Toasted Caramel Port, Flanders Red
Kegged: Hefeweizen, Centurion Falcon IPA< Dark Saison
2013 dump volume: ~2 gallons