Well, truth be told you never have to level up a second class if you really enjoy Archer (Bard) as your main. If you wanna dabble a bit you can unlock and try jobs as you like.
Mostly it's a "do what you wanna do/think is fun to do!" Try 'em all out!

All that said, there's no real rush to get to 80 to be honest and leveling a second job from 1-80 can feel like a slog, so I would say at around 30-50 is a solid place, allowing a break from the main story when you wanna just run some dungeons.

...all THAT said, I would recommend trying out the crafting jobs as..if nothing else.. you can repair your gear anywhere, including dungeons.