My LHBS just did a class on brewing with fruit, and one of the beers we tasted was a strawberry ale of some sort. Anyway, apparently the strawberries weren't ripe before they were used, and the beer wasn't very good. In fact it was bad.
Based on that, I'd wait until the fruit is ripe.
Also, crushing it or cubing it will be fine. I'd go with whatever is easiest.