I know that at sears in Canada, if you have your baby on the day that you put for your due date, you actually win everything on your registry that you didn't get during your shower. that being said I normally go look at what someone has registered for and then I go and find something with the same colours and general idea of what they want.
My plan when we go to register is to scan extra stuff, it's normally pretty easy to remove it once it's on there.
I would register for everything you want since even if no one buys the items, stores like Babies R Us have a registry buying program that give you a discount for buying items after your shower.
Also, if people don't buy from your registry (many won't), they will likely look at it and get some idea of the theme/preferences you have (e.g., color themes, etc). Enjoy!