I think all babies, not just preemies, vary massively. My LO is now 11 months old, 7 months corrected, and he has recently had a few nights of sleeping through. So many things can go wrong in a few months time like growth spurts, weaning, teething etc etc that they may do it for a few days then not, then change again. I wouldn't worry about it, do what you feel is best to get you and LO the most sleep possible. I find that sttn is one of those big things like hitting milestones etc that people talk about constantly and ask you about and try to get perfect, but as long as you and your little family is happy, anything goes.