Most shampoos and mass marketed "beauty bars" are detergent based. True soap, including Dr. Bronner's, will deposit glycerin on your hair. While glycerin is great for your skin (leaving it soft and moisturized), it can make hair tacky and give you that wild, can't-get-a-comb-through-it look

. If your hair is short and straight, it's probably not going to be an issue. My mother has been using Dr. B's liquid peppermint for 25 years! I used it like this when my hair was shorter too - the added "thickness" from the glycerin was a benefit to me.
Glycerin is a by-product of saponification (the process of combining oil/fat with lye and getting soap). Makers of mass-marketed soaps remove the glycerin and sell it. Glycerin is valuable in the cosmetics and food industries. Anyhow ... (getting back on track) ...
If you want to use true soap (including Dr. B's) on your hair and the glycerin is a problem, rinse a little apple cider vinegar through your hair after washing. The glycerin will rinse away and the ACV will leave your hair incredibly soft to the touch.
Since my hair grew longer, I keep a pint-sized spray bottle filled with ACV in my shower to use when I wash my hair with soap. (Remember to close your eyes when you spray or

that stings!). After working it around with my fingers, I rinse lightly - problem solved.
BTW, the same works for getting rid of residue when washing clothes with soap. You'll see threads here with mothers complaining about dingy clothes after washing with soap. Putting a couple of tablespoons of white vinegar in the rinse can prevent the problem by doing the same thing ACV does for your hair - breaking down the glycerin residue and letting it rinse away.