This is a little complicated to explain, but I'll try. It all goes back to the TV show The X-Files, my biggest longest lived fandom before Fullmetal Alchemist entered my life. I believe the relationship between Mulder & Scully strongly shaped the way I'd ship in fiction for the rest of my life and, at first, it was one of the things that attracted me to Roy & Hawkeye.

When looking for a new name for the fanlisting (you really don't want to know what it was called before) I ended up searching in one of the things I related them to, that is, The X-Files. I'm a sissy who enjoys meaningful subtle gestures much more than anything explicit, so I found my answer in Amor Fati — a beautiful episode of season 7 — during what just may be one of my favourite scenes ever. Mulder & Scully in a hallway (as they often were):

Mulder: Even when the world was falling apart, you were my constant... my touchstone.
Scully: And you are mine.

Years after that, I now feel more strongly for Roy & Hawkeye than for Mulder & Scully. Tragically enough, the second movie killed the ship quite a bit for me. However, I still think the name is extremely appropriate for these two. It really reflects what they are to one another, as well as their level of understanding and undeniable bond of trust between them. A straight meaning of the word would be:

touchstone \touch"stone`\, n.
a basis for comparison; a reference point against which other things can be evaluated

Roy & Hawkeye have proved, once and again, to be the other's support. Their reference point in case one of them finds himself/herself lost along the way. Something they can always depend on, regardless of their surroundings or situation.