All in all out is by far the easiest to analyze.
One aspect of scaling is that it essentially splits your strategy into several sub-strategies (although still highly correlated). That can be a crude way to increase your Sharpe/reduce variance.
Mostly however, whether scaling is good or bad depends on
- if you end up adding or subtracting alpha when you're making the additional trades (for one of the many short intraday trends, you might well be giving up most of the profit by scaling in).
- if it helps you psychologically stick to your plan due to, say, smaller losses due to scaling in slowly into a trend, or similar to take some of profit quickly.
- size traded; all in all out crossing the spread is going to strain market liquidity to the max.
There is no "right way" here, it all depends on what will work for your purposes and your trading plan.