I always go for the same amount of XP. Or if the player wants to start with less XP as a rookie, I'd find a way to give them some other tangible benefit.

Why? I sometimes play with a group that starts new characters from scratch (whether because the player's last character died or because a new player is joining the group). I really, really hate it.

In the PF game I play in the GM makes us reroll as 3 levels behind the party and I completely agree. Having players behave stupidly because dying is no penalty shouldn't be an issue if the players are really attached to their characters. I worked with my players to create really detailed back stories and motivations for their characters. As a result they are extremely attached to them and they feel the loss of a character death very acutely. It would be unthinkable to add a loss of XP on top of that. Also if I made them roll up a substandard PC it would reduce their investment in that new PC.

I meant more like, it's bad if the player just wants to get a mechanical advantage and so makes a move to kill off his character. I'm assuming one would want some level of immersion and buy-in from one's players.

Oh I completely get you there. But if a player was bored with his character and wanted to retire him in order to try something new, I probably wouldn't penalize that anyway.

I used to play with some people who talked about this guy they also used to play with, who would basically figure out a way to die every session, usually for very stupid or not-believable reasons that would make no sense for his character to do IC. Once this happened enough, the other players in those games basically told him what equipment they needed, so the constantly-dying-player would purchase it and, when he inevitably died, the rest of the party would loot and get that equipment. I would definitely disincentivize or penalize in that circumstance...but it might be more along the lines of "this is really distracting and detracting from the story, so I am not going to let you reroll for this game again."

Yeah, that's where a problem exists: when a group tries to game the system to exploit character deaths.

But yeah, if a player is bored with a character, I have absolutely no problem with just letting them make a new one that's relatively on par with what they've currently got XP-wise. If they did it more than once, I might start docking XP to, as you say, deincentivize such things.