If a show says it's currently in 'previews' does that mean reviews are coming out at the same time? Or do the reviews have to wait till opening night?

Already bought tks to Glengary Glen Ross, but just curious to see reviews for it. Hope we won't be disappointed that we didn't wait!

Reviews are opening night, unless the play was previously reviewed in an off-B'way run, i.e. as was the case with the play "Doubt". However...I defer to Howard R for the definitive and official word on this!

Opening night is press night. Previews means that they are still making changes and it is a work in progress, so reviewing it would be a no no.

I see! Thanks for the clarification.

To further answer, the main reviewers actually catch the show within the last few days of previews, but don't publish their reviews until the morning after the official opening. They do it that way, because otherwise, they would have to really rush through the writing of the reviews to make their deadlines.
In the case of Doubt, that margot55 mentioned, the NY Times' policy is not the rereview a show that originally opened off-Broadway and subsequently moved to Broadway. In that situation, the Times publishes excerpts from the original review on the morning after the show opened.