I always use a water stop and have never seen what you are describing. I use Ilford films (120 and 35mm), various developers (diafine, Microdol-X, Rodinal) and TF-4 fixer on steel reels/tanks. The only thing different about a water stop is that a small amount of developer remains active for a short time which increases adjacency effects slightly. Since my development times are always long, I suppose it makes no noticeable difference. How long are your development times?