It depends on exactly what you're using them for. Gathering and crafting are usually better uses for leves than battle. 60 is certainly a decent number to have. Keep in mind that they do cap so if you get in teh habbit of not using them you're likely to accidentally waste a few by being capped for too long. Unless you're saving them for something else I'd use them regularly. They serve no purpose once you reach 50, so now's the time.