The general rule of thumb that I've seen (I've been reading way too much about the subject), is that artists will sign their mattes in pencil to help with the archival quality. Apparently there are things in ink that can cut down on the archival life of a print.

On the other hand, it looks to me like anything that touches, surrounds, breathes near, looks at, etc. a print will bring down the archival quality. So I'd sign it however you like.

-Taltos