Open, you want some boil off to reach your desired batch size, assuming you calculated for that. Also, leaving the lid off allows for the release of dimethyl sulfides, DMS, that can cause off flavors.
Primary: Gin and Tonic IPA
Bottled: All Hallow's Eve Pumpkin Ale
Kegs: Dark Skies Black IPA Redux, Sunset Orange Kolsch Redux
2013 Previous: Sunrise Honey Rye Pale Ale, Dark Skies Black IPA, Sunset Orange Kolsch, All Hallow's Eve Pumpkin Ale, Raining Hops and Dogs IPA
Dark Skies Black IPA - 2nd Place at a local comp. Score of 39 from a BJCP judge
GT Keezer Build