IMO you want the firebox lid to seal as tightly as possible. This directs smoke and heat into the smoke chamber, where it is put to work. Too much lid leakage means you're losing a boatload of heat and smoke to the atmosphere, which is no help whatsoever in maintaining or utilizing your fire. In fact, leaks at the lid prompted me to return my first firebox in exchange for the floor model, which I knew sealed very well.
I'm not so convinced that sealing the smoke chamber is such an awesome thing. At least not with the CGSP or SnP, or other units where the entire top half opens. Here's why: The first mod most of us do is extend the stack down to grate level. When you think about it, aren't leaks around the edges of the lid essentially equal in effect?
I suppose if I had a Black Diamond type smoker, I would consider sealing the smoke chamber lid if I noticed it was leaking significantly from the top or sides.