RedShirtNo5.1
Explorer
As with a lot of the rule system, it depends on the type of campaign and players.
On the one hand, if I'm running a west marches style sandbox with a large player base of whom only a limited number will show up for a given day, I will absolutely assign xp on a session by session basis.
On the other hand, if I'm running an adventure path with a limited number of players who show up consistently, I will almost certainly assign xp regardless of whether the player attended (and it is likely that the character was run by one of the other players anyway, so was exposed to some degree of risk), and may dispense with calculating xp entirely.
On the one hand, if I'm running a west marches style sandbox with a large player base of whom only a limited number will show up for a given day, I will absolutely assign xp on a session by session basis.
On the other hand, if I'm running an adventure path with a limited number of players who show up consistently, I will almost certainly assign xp regardless of whether the player attended (and it is likely that the character was run by one of the other players anyway, so was exposed to some degree of risk), and may dispense with calculating xp entirely.