My thought is that if you're not in a static group, allocate 30 into VIT and let the accessories determine your shift in STR and VIT. If you're overgeared for content, feel free to use the STR accessories. If your healer is new to the content or minimally geared, you'll have the VIT acc to give them cushion.

If you're spending Myth though, I would recommend upgrading your "left" side first as that will have STR and VIT upgrades.

And if you can afford it, in all myth/time and philosophy/gil wise, have multiple accessory sets to allow for more flexibility.

Accessories:
Full i90 VIT accessories: 75 VIT (about 1088 HP before Defiance)
Full i90 STR accessories: 65 STR
Double melded i70 accessories: 50 VIT and 45 STR (about 725 HP before Defiance).