Install it wherever you want your modem.
It's literally the only thing that "relies" on it as the first trouble shooting question you'll get when something inevitably goes wrong, and you begrudgingly make the call to your ISP, is "could you please plug your modem into your master socket".
Obviously personal preference but we ripped out all our other telephone sockets - not used or called a personal land line in about... 5 years maybe?
EDIT: Oh, missed that you said you were putting it under the stairs - yeah, good call. That's what we've done.