I have found that the last room of the Vampire's Castle is the best place to grind for jp. I found that I am either getting 756jp or 999jp per battle. 999jp for the Magus bats and 756jp for every other encounter. These totals are with Unscathed Hero and the Growth Egg.
This is a much better range than that available in Chapter 5 Witherwoods.
I noticed that the frog enemies in Dimension's Hasp B9 give 999JP regardless of having the Growth Egg. The regular enemies give like 650-700JP. So I'm guessing, bringing the Growth Egg would make every encounter give 999.
Once you get to chapter 6 you can get a skill called Obliterate that immediately kills any enemy that is 20 or more levels below you. I ground out my characters outside Florem until about level 70, at which point Obliterate started ending the fights immediately for me. Couple this with the alarm earrings and all you have to do to grind is get into a battle, let Obliterate destroy them, and then rake in all that sweet EXP and JP. You should also couple this with the Growth egg. The reason I chose Florem is because every enemy save one is weak to fire so its easy to sweep the enemies there.
"To roll poorly once may be regarded as misfortune. To roll poorly TWICE looks like carelessness." 3DS Friend Code: Micah 4682-8451-8168