For Powers that let you skip some # of Invader Actions, how/when do you decide which Invader Actions to skip?

For Powers that let you skip some # of Invader Actions, how/when do you decide which Invader Actions to skip?

You decide when they come up.
You can skip as many as you want, up to the limit given. You are not obliged to use all skips to which you are entitled (you could always be saving them for a future Invader Action caused by something unusual). Unused skips do not carry over to the next turn (as normal for effects that don't change pieces).