Yes, shows that do out of towns are reviewed twice, but the reason tryouts are mostly done these days in places like Seattle is that they are out of the reach of especially harsh critics there. Many shows that received faint praise out of town were drubbed in New York, probably because they can be viewed as works-in-progress out of town, whereas final products on Broadway.
As has been said about Spider-Man and other new shows in previews, since the days of writing a new song overnight and putting it in the next day are gone due to all the complex technical stuff that's built into a show, the preview process isn't as valuable as it once was. Spider-Man's problem is compounded by the fact that Taymor has shown on many occasions that she is not willing to fix what's not working. In the recent interview with Tom Kitt on this site, he said that, while he was sad to see songs go from next to normal between incarnations, he always just wants to serve the piece. Taymor refuses to let things that she likes go even if they are a disaster. I'm looking in particular at Deeply Furious, which she has acknowledged the public reaction to but has still ignored.
"Art, in itself, is an attempt to bring order out of chaos."-Stephen Sondheim