The housing is very flat, if the timber is crowned across the grain due to checking etc. one can use thin strips of stock set on the edges of the member to ensure the jig is clamped square or parallel to the reference face. I think one should maintain as much contact as possible without scooping the housing. I just square the corners with a corner chisel. I fit things as they are when I'm cutting. Trying to second guess what the timber is going to do as it dries makes my head hurt. I draw bore to prevent any withdrawal. I also work on site a lot so I cut and assemble within days with minimal delay as possible. I think an assembled draw bored joint with each timber's forces working against eachother stands the best chance of staying tight. The worst part about using a router is all the sneezing! lol.
