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: Farmhouse Saison hopbursted w/ Nelson & Galaxy, sLambic II, Flanders Red, Orange Blossom Mead
Primary: Karneval Kölsch 9.0, FonBrew Brown Ale (brewed in Fondue pot), sLambic I
Secondary: Winexpert Riesling Ice Wine, Flanders Red
Kegged:Black or Blue EyePA
2013 dump volume: ~2 gallons