Stirring is not necessary for solid state fermentation... To ensure thorough activity of substrate inoculum should be properly mixed with substrate before incubation. 

While stirring is not necessary, it certainly can alter the conditions of the fermentation. In the case of aerobic fermentations, it may hasten the process along. In the case of anaerobic fermentations, it can do the opposite. It changes the aerobic nature of the system, so mixing is one of the many variables that needs to be studied and accounted for in any fermentation, not just solid state. It also matters whether the mixing is under an O2 atmosphere or another gas.

Hi, Oladapo O. Olukomaiya, continuous stirring is not required, manually shake the conical flask thrice a day to avoid anaerobic condition and provide the oxygen supply

Hi Oladapo, stirring flasks is not necesary. Initial homogenization is required, then let stand your solid state cultures for the time you need.

It depend on so many factors:condition of the cells and substrate, environmental factors , desired target of the processor etc. We should also remember that organisms differs in their growth characteristics and in the production of primary and secondary metabolites and conditions responsible.