Apart from the stuff mentioned already, I find it's pretty helpful to get way of the bow (?) for fire arrows. Not overly important on land, but they can really turn the tide at sea (har har). Bow Kobayas are noticeably better with them that's for sure.

Generally, I go for arts that open access to something, be it buildings, abilities or recruitment options, rather than arts that provide small boni like experience for recruits.