Bob D659 does, indeed, bring up a valid point in that the development process does release gas. But, so does carbonated soda. That is why there had to be a truly leak proof container made for soda. Enough did not 'complain' that the same should have been done for 'mere' darkroom nuts.
And, Ken Nadvornick, there is a valid point here about the desirability of using a material such a stainless steel that AIDS in conducting temperature (ie, water bath). But, still, there are some who would like a truly leak free tank.
And Gerald C Koch, I am not saying that you are wrong with your SS tank with plastic top being entirely leak free, but I would like to put it dry upon a towel, without your water bath, and see it stay dry. I have my doubts but it might be as you say. In a water bath you would never notice a leak. And you could not prove this by merely putting food coloring in water to see if any color came out into the water bath. As I just confirmed Bob D659: there is a pressure build up that 'forces' leaks in the development process. - David Lyga