Idling in Team Fortress 2

You will not be punished for joining a server running "achievement_idle" and leaving your computer on.

However, the marginal gain idling provides means that it's not a very effective way of getting items, especially if running your computer for that long means overheating.

Valve never "punished" idlers, only those who used the external program developed by Drunken_f00l to trick the steam servers into thinking they were in the game, connected to the server, when in fact, they were only connected to the server, outside the game.

For what it's worth, I believe Valve stated that ~1 hour a night was enough to get all your weekly drops.