Hi sorry if this is a silly question but I have confused myself 
At what stage do I retoplogize? Do I do it after I have made my model as high res as I can go with the details? 
Usually, you would do it before you do any high res sculpting. The reason for retopology is to even out polygon size so that most of the ploygons are of equal size which gets rid of stretching and also for good edge flow which will also aid in sculpting the finer details.
Thank you Zber2 
I had just watched a ton of Zbrush tutorials and lost my way-easy to get distracted by trying to learn everything!!
Your welcome! I should also mention that ZBrush is partial to square polygons, as opposed to rectangular or elongated polygons which will also avoid stretching problems when sculpting fine details.
Aha! Squares.
So I make my base mesh-retopologize-then sculpt fine details-then pose the model-then decimation master?
So much to learn!! ha ha