I'd say compare the process to any software update you might do on your computer... You always gotta check for bugs on your particular hardware / OS setup. I never update crucial software (such as Logic Pro for music production) on my Mac without checking other users' experiences first!
I guess most Apps are a little different, as most would probably fall into the 'non crucial' category E.g they aren't gonna wreck your life if they don't work... For Apps that are crucial to way you work or live, investigate first. For Apps that aren't, take a chance. If it's a free App, you can't really complain about anything (particularly advertising)! If it's a crap update, rate it as a crap update.
The iTunes review process isn't a bug fix process for developers to sponge off, it's to ensure that developers develop within Apple's framework, if that makes sense...?
peace!