How to care for a DSL modem

This question does NOT have anything to do with Vista. Because I cannot find
a good place to ask this question, I am trying it here.

The question is as the subject line: how to care for a DSL modem? Should I
turn it off after I am done with an internet session--or at least during the
night when I am sleeping, or should I leave it on all the time?

I live in Los Angeles and am using the ATT Yahoo DSL. One of their tech
support guys told me that I should power it down when I am not using it,
because the heat that is otherwise caused can shorten the modem's life.
Another of their tech support person told me that I should leave it on, and
he has related to me that is what he has done and that his modem has lasted
him six years now and counting!

So what should I do? To leave the modem on all the time, or should I turn
it off after an internet session or at least during the night when I am not
using it?

Hope someone can drop me a clue here! Thansk in advance!