I'd keep the lid on to protect from dust...
I've got a fully loaded chasis in front of me here and have never had the fan running. Never noticed a heat issue (not hot to the touch).
IIRC, I was told by the folks at Lavry to not worry about it unless your in a very warm room or the unit is next to other gear that runs hot. You would be wise to confirm those statments rather than take my word for it. E-mail the tech support at Lavry.
Fanless,