Personally, I'd put the Power Tabs on Magus; he doesn't need the Magic Tabs, his Magic levels cap even before Lucca.
For the magic ones, it doesn't matter that much, if you ask me. Luminaire is ridiculously deadly even when you first get it, and later it's the single most powerful spell in the game. If you want to sear everything to a fine crisp as early as possible, put the tabs on Crono. Otherwise, if you plan on keeping that particular team, force-feed them to Robo, who should also get any and all of your speed tabs you happen upon by now.