Is it personal preference or code to connect grounding wires and neutrals to separate buses?
At a sub-panel, the separation is mandatory.  

At a main panel, it's optional because the neutral and ground bars are bonded, and are allowed to be the same bus, so it is legal to blend them together.  However... If that panel is ever made a sub-panel, separation makes things a whole lot easier, not least, it assures you left enough wire length to reach the right buses.  

Separating neutral and ground also reflects that the person actually understands what they're doing, rather than simply blindly following recipes by rote.

The core concept is that Neutral Is Not Ground.  They are completely separate things which must be isolated, except in the one place they are intentionally bonded for a specific purpose. 

So when you see Irish spaghetti** on the neutral bar, that tells you the installer doesn't really have the reason for the rule at the front of his mind. 



** Irish flag being white, green and orange(copper)