What a fantastic idea! If possible, I would get the blocks or signature area together as soon as possible and then as you see family or close friends before the wedding, get them to sign then. Make a list so you know who has signed a block. At the actual wedding/reception, you will just need to get those people you didn't see to sign....out of town guests for example. This would give the guests the opportunity to give some thought to what they would like to say, you would have more control over the placement of the signature, and it would save some time at the actual event and maybe not miss someone special. Enlist the help of someone on the groom's side to get his family/friends signatures too.