This is true, but it all depends on availability. Altanta to Newark in the AM is great because they have lots of flights, about every hour or so.

I've often been booked on a sat morning early AM flight(atl to ewr), checked with the onepass folks before going to bed, discovered lots of seats on later flights and decided to sleep in a little later, never had a problem, never paid a penalty.

A few times when I checked in with the ETM's it gave me the option of switching to a later flight.