Quick question that came up in a recent tournament... When using stunning blow feat. does it have to be declared before making ### attack????

Generally if you don't declare Incap is off, then it's also assumed you are using ### optional (i.e, on unless turned off) feat card. If you're using a figure like V Nebula, you should definitely be saying "I'm using RCE..." or "I'm using Neb's Incap..."

From ### Icons Rule Book:

A feat assigned to a character is in effect during an action unless ### player who controls ### character indicates at ### beginning of ### action that ### feat is not being used. ### feat resumes its effect immediately following ### completion of an action where it is not used.

So you only have to make a declaration when you are NOT going to use a feat.

I think it's just easier if you delcare what you intend to do before you make any rolls. I've seen people, and played people, who roll first and then decide what it was for depending on ### outcome of ### roll.

Um I was attacking fig A cause my roll missed fig B. Oh, my guy has RCE? Of course I used it to attack you. Oh wait, I hit you but you have imperv and I can only do 1 damage? Well it was an incap attack then.

Say what you want to do and make sure your opponent is clear on what you are doing, that way you can avoid any misunderstandings and bad feelings.
I know I don't like playing people who I think are trying to pull a fast one on me.