I've found that carefully placed shot can make controlling otherwise slack line while shipping out a lot easier. I'd be wary of putting much weight much further than halfway down if you fish with a very liquid paste - the weights momentum could cause the hook to break out. I'm relatively new to paste fishing, but I've asked a hundred questions and gained a load of good advice!
The float will take the weights momentum if the shot is halfway to the hook (or less). If you are getting shot bites then all the weight immediately below the float makes obvious paste-saving sense!