My ducks lay early morning, usually I don't even seen them lay and they're let out around 7 in the morning. I've also heard it depends on the breed of duck. While some breeds make a nest, I've read that runners and some other breeds (who's names escape me) will just lay their eggs in the field and walk along. I honestly can't say if that's true or not but I read it from the 'Keeping Ducks and Geese' which is a published and fairly popular book.
No matter what breed you get or when you let them out, you can't really disrupt their laying to the point they won't do it. (At least I've never heard of that happening) Chances are though, if you let them out too early, they'll lay in the yard. Not THAT big of a deal unless you bought your birds for the purpose of getting eggs and even at that, as long as your yard isn't HUGE you should be able to locate them. Have fun with your ducks! They're a lot of fun!