I prefer to be as transparent as possible, mainly because I don't feel like the game should be GM vs Players. Obviously story and adventure-related lore and details are kept under wraps until needed, but I let quite a bit of GM information hit the table in advance. Anyway, I have a few guidelines...
1. If it's in a published book, I just assume players have read it. There's really not much reason to try and hide stats in the MM, for example, since it really only works once per player. The only exception here is if I'm running a published adventure, since I don't want stuff ruined beforehand (if possible). That said, I still usually do a recap after each session or maybe every few sessions, giving a behind-the-scenes tour of what I was doing, and the how's/why's of things I might have changed from the official adventure. That leads me to the next one...
2. I'm bit on pulling back the GM curtain to give my players an idea of how it all works. I've found it's a great way to introduce players into the GM role, without making it super obvious. They get to play, and learn a bit about how I do it, including the decisions I have to make, or what I struggled with, etc.. At best, someone will want to give it a try at some point, and at worst players might be able to metagame in a favorable way for once.
3. Despite having a reputation for transparency, most of my players generally know that I can and will change things to fit the story/balance/vision/etc.. In those cases, I'm not always as transparent about the details, but it's pretty obvious that I've changed some things around and they tend to be a bit more cautious until they figure out what it is.
Anyway, I guess I just really subscribe to the belief that RPG's are meant to be cooperative games of both conflict resolution *and* storytelling. To that end, I prefer to avoid feeling like the enemy.