In this situation (since it was folded, or generally not observable upon opening the card) I think it is more appropriate to look at the amount later. It goes further to avoid looking grabby. Not that one automatically looks grabby if s/he does choose to unfold it, but...ok here's what is going through my mind -why would someone fold it to put it in a card, perhaps they didnt want the amount seen by others, or perhaps it is just habit. But then he said to open it right away, which makes me think he did want others to see it, or to see your reaction to it at least (this could be from pure or not pure motives).
Either way, at a rehearsal dinner, the point is to have people celebrate with you/thank them for participating, not to receive gifts (as opposed to a shower where you typically open gifts in front of guests). So I think waiting until later would be more appropriate. Just because people so commonly look at the amount given, doesnt make it proper to me.
For example, my BF told me a few weeks ago that he received an unexpected bonus from his boss. I was suprised and happy for him, and congratulated him on his accomplishments. He was surprised that I didnt ask how much it was. I told him that it didnt matter how much it was, because I was just proud of the fact that he was being recognized for his work, so it didnt really matter how much it was. It's the same principle - one is just happy that people are celebrating their commitment to each other, not hoping for gifts to commemorate that occasion.