I've been using the 1 tab per 20 gallon rule and it works for me just fine. I've read higher doses can be used in wines, meads, etc to kill off wild yeast from fruit before pitching cultured yeast.
If you're using sodium metabisulphate vs Potassium metabisulfite, too much would effectively add a quantity of potentially undesirable salt to your water.
I'm not overly concerned with extra salt as 1 tab per 20GAL is a very small amount.