I think what you mean is whether it is NECESSARY to keep those filters in the dry cabinet right?

Filters are relatively cheap compared to lenses unless you buy very expensive CPLs. As such, in my opinion, it is not necessary to keep them in the dry box. In addition, I don't think it is easy for filters to get fungus as they are just a one piece part.

Another example is your typical compact camera. I don't think any typical user would buy a dry box even if the camera cost RM1k ............ 

For RH% setting, during my research, there were varying recommendations regarding RH% setting for DSLR. As far as I can tell, most experts would recommend an RH value of 40-45%.

The reason to recommend 42-43% is because of small inaccuracy in the boxes i.e. you set at 40% and your dry box might be running at 38%!!!!