For me i generally keepped as an my SL at 20-30 pips when i am sure about the trade and when i am not sures of the traded, i setted as it at 10-15 pips because that saves me from taking much as risk. I generally set my SL before my trade is executed or after its execution. I never modify it after i have set it once because mostly i set my TP and SL and then close the platform. Its safe and doesn't generate greed !!