I usually depict any river that is important to the purpose of the map. Rivers that influence travel and navigation should be shown—if it has significant riverboat travel or is too wide and/or deep to be forded near most arbitrary locations. If it has a large impact on settlements in the area, then it should probably be shown. If it defines a political border, then it should probably be shown. A map of the physical geography of the area will probably show more rivers than a political map.
It may also be wise to keep several sheets or layers of rivers with different levels of detail in a master document so you can quickly generate different kinds of maps with the level of detail you need.