Please do it, it's primarily for bug #273008(you can test 0.8.0 by looking to see if xvid sees the yasm version as 0.8 and succeeds to compile). I already did amd64...
At least for x86_64 configure for xvid prefers yasm over nasm, so I've adjusted the dep in xvid's ebuild accordingly for amd64. Please see which one is preferred for x86 (you can see that in xvid's configure output), but I assume it's using nasm primarily for x86.
checking for yasm... yes checking for yasm version... 8
x86 stable, all arches done.