I don't understand the question. Are you saying you have to sulfate shampoo because of buildup but then you sulfate free shampoo the rest of the time? And are you saying you cowash occasionally and that seems to be beneficial? If it is, then I say cowash. It makes your scalp happier. And maybe you could post your whole routine and what stylers you use so the experienced curly girls might be able to pick out what you're using exactly that is causing the buildup. Then you could drop those buildup causing products and cowash more often.
3A - C, HP, ME, HD. (Coarse, High Porosity, Medium Elasticity, High Density.)

CG since Nov. 2012

Poos: SM Moisture Retention + Yucca Baobab, TJ's Tea Tree Tingle
Condish: TJ 'sTea Tree Tingle*, SM Moisture Retention* + Curl & Shine + Yucca Baobab, Yes to Blueberries
Stylers: KCKT*, SM Curl Enhancing Smoothie* + Curl & Style Milk*, KCCC*, FSG*, CJ Pattern Pusha, Curl Keeper
Sealers: Jojoba* or Grapeseed* oil

* = HG