Will I need to increase my washing frequency to get dreadlocks? - Dreadlocks FAQ's
Many people without dreads wash their hair each morning as part of their routine. Switching suddenly from washing each day to washing every 3 days often causes an itchy scalp. For this reason it makes sense to prepare the scalp by adjusting your washing routine a couple weeks prior to dreading the hair. This lets the scalp get used to the new routine.
For those that do not have dreads yet and are washing their hair every 4 days or every 5 days, either because their hair takes that long to get oily, or because they have a dry scalp condition, there's no need to increase washing frequency to prepare for dreads, but it is an option. It's true that the dreads would probably mature a bit faster if you did wash them more often, assuming no residue is left, but it's likely that washing more often will bring on a dry condition, especially if it's done suddenly. If you plan on increasing the washing frequency to see if the scalp adjusts and produces enough oil to not get dry, you should do it a couple weeks before dreading so the scalp has plenty of time to respond. This would also be a good time to change over to a residue free shampoo so that after the hair is dreaded the only thing your scalp will need to adjust to is the dreads themselves, which is usually not a problem, assuming the bands at roots are not overly tight and you are able to stimulate the scalp and help the skin exfoliate.
That said, I would not recommend extending the washing frequency beyond 4 or 5 days. I know some people only wash their hair like once a month, dreaded or not, and while it's certainly their option to do so, it's definitely not ideal for helping dreads form (it slows the process down a lot) and God forbid you get stuck in an elevator with them on a hot day. #bygones