In my very first RP here, and my first forum RP ever (I'd been a table top RPer only prior to that) I used another player's NPC brother in a post of mine without thought, it just fit so I went with it. My fellow players had fits, telling me (mostly nicely) in the OOC that what I'd done wasn't ok. I was mortified but the player whose NPC I used was very cool about it. He said I'd stayed within character of the NPC and he was fine with it but to ask in the future. As the game died shortly there after (vanishing GM) I never got to give it another try. 

I think with NPC's there are lots of shades of gray to the matter. As a player I am (as demonstrated by my fumble above) willing to take on NPC's and work with them once I am comfortable with the setting and the NPC. This is easier done in a 1x1 when things are communal from the start. It gets a little trickier in a group. 

I know when I GM there are NPC's who are plot critical and I don't want anyone moving them. However having to play every single NPC in the RP can be exhausting. I've had an RP turned into a series of 1x1's several times as players squared off with their own custom made NPC while that can be great fun, it can also be draining if they aren't also seeking each other out which is something I try to encourage. So I try to let it be known which NPC's are hands off and which are free to be used by any and all. Throughout the course of the RP such statuses can change which further muddies things. I have also had many players bring in NPC's that they either played or expected me to play as the GM. Sometimes it worked, sometimes it didn't. 

So I guess it really comes down to communication. As that kind player said to me so long ago, just ask.