what happens to your contrib value, if a game turns up in a bundle later on?

No.

As far as I know, as long as you create your giveaways before the date that those games go on a bundle, you should still get full contribution values (which is equivalent to current base price in dollars on Steam Store)

Your CV will drop as the games prices drop

If it appears in a bundle later on, your CV is safe. But it is lowered if the base price is lowered in Steam (i.e. game cost without any sales)

You still get the full points if the GA is created before the bundle.

I gave away games like Frozen Synapse, Anomaly which later turned up in bundles and got the points.

awesome thanks guys!

But remember that the bundle list is retroactive. So if they decide Tomb Raider and Just Cause will go on the list, whether you make the giveaway now or later won't effect that. On the other hand, if they don't do that and it goes into an actual bundle, then you should do it now.