It's possible to add camera effects like motion blur, depth of field in post. Doing so gives you much more flexibility.
But of course its easy to add these at render time.
My question is, when does it make sense do render motion blur, dof, vs add them in post?
Are there any guidelines or best practices?

Adding DOF and Motion blur at render time increase render time. It is more common just to kick out a companion Z-depth and or velocity pass along with your beauty pass. This avoids any re-renter time if you happen to have a setting wrong in either DOF or motion blur. What if you want to do a rac focus after the fact? Having these passes available in a compositing package allows you to tweak the settings more interactively and respond to change requests quickly.

I rarely add motion blur or dof to my renders unless it is just piece for me.

Thanks!Good info.