quote from dottom :
I never scale in or out. The only reason I would consider doing so would be psychological reasons. Too many traders scale not realizing that they are in effect utilizing different systems. If you buy on a breakout... then scale in some more on a pull back... then you are trading two separate systems. If you enter all your positions on entry, but scale out some for small profit and let the rest ride with trailing stop, you are again trading two separate sytems.
Each system has its own performance and risk characteristics. If you want to trade both systems (e.g. both a breakout and a pullback system to even out the aggregate equity curve), that's fine, but realize what you are doing when you scale.