Personally I think that unless I saw some reason to do it, I wouldn't do it.. I don't know. Tough call. Everyone's going to have their own comfort level on that I guess.
Not that it sounds like you need further convincing (but since I like talking about fish
) .. FWIW, unless the eggcrate is weighted down, they can still sometimes get out. I've had fish jump and make it out despite eggcrate being on the tank at the time. I lost a 4" tang out of a QT tank in the fall (and there was an eggcrate cover at the time which got bumped right off the tank). Also had a wrasse jump out of my display tank but I have no idea whether he lifted the eggcrate and it fell back in place, or whether he shot through the square holes and just happened to fit just right.