Exactly how this is handled can vary with local rules, rather the game is cash or tournament and sometimes depends on rather or not the game is rake or time. The most important thing for you is that you decide how it is done and do it the same way in your game as a matter of procedure.
Here are the ways we do it at a large casino on the Las Vegas Strip, with nice fountains:
Tournament- All stacks dealt too, if player is not seated hand is dead. This is the way WSOP does it and pretty standard when it comes to tournaments.
Cash Games that are racked. If the player is not in seat or near they are not dealt in. It is OK if they are walking up, or standing nearby to deal them in.
Cash Games where players pay time. All players dealt in rather or not they are there. Mostly the same as tournaments except the hand is not automatically dead if the player is not in the seat. The hand is killed in turn. If the player is coming and the player on the left has not acted, the dealer will try to let the table know the player is back.
You really want to achieve a balance between game speed, security, and how casual your particular game is. At the WSOP and other tournaments you really want to be terse with a rule that says your hand is dead if your not in the seat, and enforce the rule evenly, with the floor person ruling dead hand every time. The time game vs rake game is just something we do were I work and I have no clue if it is done at other places. It has up and down sides, the upside is that it keeps some time players happy. The downside is that if you brain fart and deal around the empty seat it is a misdeal. The important thing here is that whatever you do is reasonable for your particular game. A more casual home game is about having fun and constantly having your hand killed on a technicality while you get a beer is no fun. And of course having some recourse against a player that is always someplace besides the game and expecting to be dealt in and waited for is also a useful thing.