Hi Karen, my baby is only rolling to get where she wants now, and all her friends are crawling. I think I've heard that babies commonly roll anywhere from 4 weeks to 9 months?
From what I understand, rolling, crawling, walking are all extremely variable 'milestones' in healthy babies. It is great that she does everything else appropriate for her age... IMO it is more important to look at communication (hearing, speech) as this is just so important to our social development as people, and an area where early intervention if needed is really helpful.
Having said all that, there are great excersises you can do to encourage the rolling, and of course giving lots of tummy time and floor play will help too. (I have been a bit slack with all that - having a baby that cried a lot, for my own sanity I have carried her around a fair bit. I guess I'll just have to live with that if I've delayed her gross motor in some way...)
Hopefully you've gotten the point of my babble - your daughter isn't necessarily 'late' to roll (and for a premmie, you'd be a bit more patient as she had a rough start). My DD isn't the best example to give you, I know, as she's now 'late' to crawl as well and will probably be 'late' to walk, but just wanted to share my take on it with you - and hopefully it has been reassuring, LOL!
Incidently, my nephew (who's mum, my SIL is on here!) didn't crawl til 11 months and walked soon after, and has brilliant gross motor skills as a toddler - he's a really fun and active little boy.