TOPIC: Input needed for Player's Guide updates

You handle it because players manage their own HP.

If the DM is not busy dealing with other things at the time, feel free to notify her/him, but it's not required. If the DM is simultaneously trying to manage demands from multiple players, there is no reason to burden him/her with it.

I would say that while the player should definitely handle damage reduction on his/her own, the player should let the DM know about immunity (not at the start of combat, but if they are attacked and take damage to which they are immune).

I think it can't hurt to tell the DM you have reduction for any of it, I mean, we get to hear the monsters have damage reduction on our attacks...

And having seen people's builds, many of the forumites are geared up monsters.