DO it however you like. However doing it in Ableton offers more flexability, since in Ableton you can easily change the panning and get it how you want but if you decide the panning in Ableton from your Cubase panned loops are wrong you are going to have to jump back into Cubase, recut the loops, then go back to Ableton which is a lot more tedious than just tweaking a knob in Ableton until it sounds right.