You can evade while guard is up. it's kind of annoying actually... I usually hit phalanx before I realize I didn't "block," I suppose, overall, its best that I didn't take any damage though.
Parry can't happen while guard is up... but unless you are a robot there will always be those occasions where you get hit just as guard is falling, before you can put it up again. I notice I parry a significant amount during those small windows of time. I wouldn't go out of my way to maximize parry, but it still adds to damage mitigation overall...
Personally, I like to keep my dex just a little over my str, but mainly focus on vit. I play more than GLA though, and I don't like having to reallocate if I can avoid it.
FFXI: Ferth - Cerbereus Nee Hades Nee Leviathan.