No probs, What I generally do is use a full cube delete half of it then make a instance (same low poly level) untill I get a decent edge loop folw etc, i'll then delete the instance and the history on the low mesh then make the sub div proxy, so I have a half smoothed half low poly model, that way I just make the changes on the low poly model and it's updated on the higher poly model.
Hope it helps and makes sense!!
edit- I generally make the sub div proxy a negatave in the scale that I need, so it goes on the other side.
"No pressure, no diamonds" Thomas Carlyle