Depends on the weight you are towing, if vehicle is behaving well in drive, don't bother, sport mode or even holding in lower gear is always good on hills, up and down, ie engine braking!!
What I find is that sometimes on a long motorway hill the gearbox will hunt between 3rd and 4th. Sport mode moves the shift point so if that's happening it's a bit more relaxed. Or holding in 3rd has much the same effect.