You add to strong intraday trends and only when all of your current positions are showing a profit. Either that or you go all-in / all-out. All other roads lead to greater odds of ruin.
If it goes against you and you can get a better average price ... why is it better not to do it? Is not it really an advantage?
Logically... The decision to apply martingale or not should be based on analysis of your trades: If applying it yields a significant increase in profit (with a reasonable level of increased risk), then consider using it.