I believe rest days may be required for some parts of the penis, and “rest periods” for others. The former
depends on your bodys’ capability and type of training you do.
If one subscribes to the micro tear model of PE (small tears in the collagen fiber matrix, ie; sprain), then
causing more micro tearing in the connective like tissues of the tunica through more frequent training
does not seem likely it would cause lack of measurable progress, however it may make large scale
structural failure more of a possibility if more “un-healed” tears accumulate.
It does appear over time the tunica becomes more resilient to these stresses, and it becomes more
difficult to create these tears. This might be perceived as a plateau caused by over training, but in fact
the connective tissue have become toughened and must be “deconditioned” through a period of no
training before further progress may be realized.
Most methods of causing this tearing, such as jelqing, in one way or another tax the vascular and neural
components of the penis also. It is for the repair of these tissues that rest days may be required.