PK/PD modeling - fixing population paramterers vs. fixing both pop. and individual parameters

Hello NONMEM Team, We usually try to separate PK and PD models and the reasons are obvious. There are different ways to separate the models. Are there any good references which show advantages and disadvantages of fixing population paramterers in the PD model ann allowing the model to estimate individual PK parameters vs. fixing both population and individual PK parameters? Thanks! Pavel