I think that this might be an important question to ask ourselves. There is no right answer. A lot is situational. What is my PC like? What is the other PC like? What are the current circumstances? What kind of advice/instructions am I giving? All important factors.
But I want to propose another factor... How can I present this in a way that will encourage the other player to be themely, take risks and push story? And this is a question I encourage staff to ask themselves as well, not just players.
I think there are few things to consider here.
It's easy for players to take the instructions/advice they are given as holy writ. The next best thing to an OOC rule if not mistaking it for an OOC rule entirely. Or a statement of fact or truth about the game world that can never be challenged so why even try. I have seen this happen several times and it often doesn't just stick to one player. All too often it gets circulated until a significant portion of the player community believes in such non-existant holy writs, facts and truths that can not be challenged.
The other is how we can each do a little to train the player behind the PC, a little bit at a time. I think it's super important that players know that any and all IC laws, rules, demands and instructions are optional in a sense. You can go along with them or you can choose not to or you can try and change or destroy them. They are simply factors to be considered when making your moves. Choosing not to go along may come with consequences that you have to consider but you can still do you.
I realize that engaging other PCs with this in mind will often make the interaction longer and more complicated. Sometimes it might even seem contrary to our PC's own interest. But I think that if we all try and consider these things when we interact with each other, it could help a lot of players get outside of the boxes many feel they are locked in. It would help promote more conflict that I think many would enjoy.
And it doesn't have to be something you slave over every time your character opens their mouth. I do think that it comes more naturally over time, to the point where you can do it while still pushing your character's own objectives. But if you just put this into play every now and then I think it will help other players a ton.
I don't think these things are as self evident or obvious and many thing. It's especially easy for old timers to just assume that other players should and do know these things. Or that they will figure it out on their own eventually. Or that they can go read the BgBB to learn some of this. But I think that the best way to help other players learn things like this is in game as they play.
Some lame examples:
"Don't go into the sewers if you aren't WCS." vs. "Sewers can be real dangerous. Creatures in there will try and eat you. The water levels shift rapidly. You could drown. WCS considers it their turf."
"Corpies are not allowed to go to the mix. You are a corpie now." vs. "Don't get caught in the mix in a way that makes the corp look bad. That leads to trouble."
"Pay Jorge, Smith and Jen a kay every week. It's the law down here." vs. "Sometimes someone will demand you give them flash and will try and come at you hard if you don't comply. Choice is yours but know there's a good chance it will happen."
"Don't ever go in there without a guide." vs. "It's strange in there. Dangerous types with their own rules. Bad things can happen to a guy who goes in there ignorant or without a guide to teach them. Seen it done before though."
In these examples, the first version is common and even reasonable advice. But they are easier to take as unwritten OOC law and don't do much to encourage characters to see them as factors in decision making and not roadblocks. In the second I tried to convey the warning/advice but in a way that I feel is less likely to be taken as HOW IT IS, DO NOT CHALLENGE. I try and suggest to the character and the player behind them that they can choose to ignore the advice or warning but will face possible consequences.
Honestly, these are weak examples but I hope they convey the general gist things. I am not trying to tell anyone how their character should speak or what their positions on IC issues should be. I'm just trying to solidify the somewhat mercurial thoughts I was trying to convey. Everyone should play how they like as long as they are following the @rules!