If you see a really good deal I would buy now, and either take it off your list and replace with something else or you can just leave it on the list and have extra or if someone does buy you can always return and exchange. I say buy because you never know people may not buy everything on your list.
I agree with this!
Most people don't have their hearts set on buying you any particular item so it's no skin off their nose if your registry changes. (Heck, I go to the store, print it out, buy something in my price range, & leave.) So if you see an amazing deal, get it, and people will get you something else.
I ended up doing something similar when DH & I found a good deal on a bassinet for DD#1 a month before my shower at a different store than where we'd registered, plus a coupon for it at that store. We knew it was a bigger item, too, so we figured we're better of getting it at a great price would be smart, rather than having someone who was so inclined to buy a bigger present for us spend more than they had to for it.