For those of us like myself who are versatile and are willing to switch roles and love variey, how do you decide each time who will be the top and bottom.  Is it simply up to the client and the provider goes along or is it more of the spur of the moment dynamic.  

I tend to go by intuition and ride with the moment and according to how both of us feel.  Usually whoever wants to top does it.  But I'm curious how others decide. Thanks!