Best available versus need

When do you deviate from best available to need? In mock drafts, my board is usually 2 or 3 RBs in first three rounds and another in the 4th. Receivers are usually 4-8 spots lower. Do I take the drop?

I usually take best available for the first 5 rounds unless I’ve taken the first 4 at one position and then I will go the opposite way. Once I fill that need in the next few rounds I once again go back to best available.