I wash with only water and treat my scalp once a week with a rosemary solution, which is basically 3-4 dried rosemary branches (or about 3 tbl spoons of storebought) with 2-3 cups water, bring to boil, simmer for 10 minutes, dab on the scalp with cotton or paper towel, this should kepp your scalp healthy, i get dandruff/seb derm every autumn/early winter and sometimes I have to use Nizoral shampoo, it wont harm your dreadlocks as long as you only apply it to the scalp where the problem is...dont use for more than a month but should be avoided.
btw wash with water but get your fingers in there and scrub especially important if you live in the city as the cause of some scalp problems and dermatitis is from the grime in the air that builds up when you dont shampoo and comb everyday
if you do shampoo just pay attention to the scalp, no need to wash the actual matted locks, it causes buildup even with res free shampoo, they get clean just from the excess shampoo making its way down
hope this helps... if not pm me