To me, the updates break down into 2 categories:
* Things that will automatically sort themselves out in the CB and MB, and
* Things that I need to know.
I skim through the stuff that will be in the CB or MB each time new updates will be released, but for the most part I just rely upton the tools (and Compendium) to give me the updated version.
For things that do not impact a monster, power, feat, item, class, race, etc... directly, I give them a more careful read and keep a sheet of notes for things that seem like they might come up in my game. I'll either email players about rules that might impact their PCs, or just mention the changes before the start of a game.
If you stay on top of the rules by giving them 15 minutes of your time every few months, you don't have to go through the entire document every update. You can just read the front section for each book that lists the recent changes.