I'll put it to you this way. I've had two or three occasions upon which I really felt like I had to step in as the F/O because the Generalisimo was doing something stupid. And I did it relatively pleasantly. "Oh, uh, hey, shouldn't we maybe turn the radar on?" etc. Or, memorably "Uhm, hey, you know the flaps are still at 15, right" (as I put them up and we accelerated through flap speed +10 or so). On the other hand, I've been "corrected" repeatedly by F/Os with a quarter of my my time (and near as I could tell, very little relevant "experience") on things that were really just a question of personal preference and didn't exceed any limitations or break any laws more times than I can count. I responded pleasantly to those, too. In some cases, I just did it their way (which was also totally safe and reasonable) with an offhand remark about how "gosh, I didn't see that in the AFM" and a laugh. In other cases, I said something like "nah, I'm probably going to do it this way because (insert reason)".
There aren't any hard and fast rules about this stuff, and there shouldn't be. That's why there are two real human beings up front...it works. But as an F/O, my first instinct is to say "hmm, now why is he/she doing it this way?" rather than "THAT'S NOT HOW I WOULD DO IT, LET ME SHOW YOU THE RIGHT WAY". Come to think of it, that's my first reaction as C/A, too. I guess my point, if I have one, is that in most circumstances the person flying the plane is doing it the way they're doing it for a reason. It may not be a good reason, and if it isn't, if I'm the C/A, the time to talk about it is on the ground unless it's an imminent danger to the flight. If I'm the F/O, the time to talk about is probably never, again provided it's not dangerous, just inefficient or, what, "irritating"? Does that make sense?
If you think about it simply as a question of Safety, good. IMHO, Safety is compromised far more often by one guy (whichever seat) critiquing everything the other guy does than it is by one guy rolling his eyes privately and letting things work themselves out. CRM doesn't function if every action becomes a member-sizing contest.
The most dangerous (two pilot) situations I've been in (at least that I can recall) were when one or the other of us was pissed off at the other one and our egos were on the line, somehow. Check em at the door until it's Serious, or so says I.