Depends on a few factors:

Choose if you want to pull on the banner anymore (e.g. you pulled Squall EX but missed Selphie's 35cp).

If you already got everything on the banner but would still like duplicates (you've gotten all weapons, but you haven't reached 4 on any of them).

Once you've decided that you aren't pulling anymore then yes it is usually a good idea to MLB the 35cp if you have that characters' EX. Squall/Tidus/Noctis/Terra off the top of my head all benefit greatly from their 35cp passive. On the flip side you could wait until that character comes around in a weekly banner (as their 35cp will be on it) or their World of Illusions (for Squall). Personally I have a surplus of powerstones at the moment so 1 of each weapon was good enough.