I agree with hairhealth (above): other products can build up besides silicones. I stopped using products that contain polyquats when I discovered that cowashing was not enough to remove them (from my hair; this may not be true for others).

But even being polyquat and silicone-free, I still do a low poo at least once every two weeks.
Medium/coarse, low porosity, dense, mix of 3s
CG (again) since Nov. 2012

Cleanse: SM Moisture Retention, Elucence MBC, KC Come Clean
Condish: Giovanni DL, Cure Care, Ion Treatment, GVP Balm, AOHR
Stylers: KCCC, SM Souffle, AOMM, SS CEJ, JCRR, JCCC

Trying out steaming!