My thread along these lines from a few years back:
http://www.tfproject.org/tfp/showthread.php?t=90323
The thing is, most people don't want to be wrong, and most people don't want to be corrected.
The only way to have an opinion and not insult others is to (a) decide whether it's even worth telling them what you think (b) if you do, give them an "out". Give them somehow that they're still partially right. They'll internally know they're wrong, but they won't have to be called out in public.
Most of the times, A will stop you. But if they're really wrong, really making a bad decision, or making a decision that could have dangerous consequences, it's worth it to let them know but still give them an 'out.'
And unless you know the person well enough to determine how they handle embarrassment, being wrong or being corrected, it's NOT WORTH IT to 'instruct' them.