Rant: Just because you're the pos 5 doesn't mean the tome is necessarily for you.

Before you buy a tome, and especially the first 10 minute one, PLEASE open the scoreboard and look at your teammates' levels and consider who needs the xp the most. For many pos 4 heroes like Clock, Nyx, Lina, Sky, Puck, etc, the difference between being lv 5 and 6 is absolutely huge, because their job is often to make plays and without their ulti they can't do that and are forced to inefficiently farm a jungle camp or push a lane (meaning they show on the map as someone who should represent kill threat so the enemy isn't afraid and can make moves on their own). I had a game where I was lv 4 Nyx and the pos 5 Rubick who had had an easy lane and survived a teamfight was already lv 6 but still took the tome. This hurts your team really really bad and makes the 4's job much harder than it needs to be. And this is coming from someone who mainly plays pos 5 btw.

Later on in the game, when the 30 and 40 minute tomes come along, it can also be better to give it to someone on the team who's close to an impactful lv 20 talent for example. Letting a core go from 19 to 20 is more impactful than having a support go from 15 to 16.