Multiclassing

I recently took advantage of the events to class change some of my characters, but I’m thinking I might have done it prematurely or should continue training before switching again. At least with Rinne, I switched her to thief but it seems that after Lv32 there’s no much reason not to switch her back to Ninja. I have Adam as a Priest Lv35 and Alice as a Mage Lv37. Should I keep them in those 2 classes permanently or level them up to a certain level and switch them back to their originals? I also have Linaria as a Ranger Lv31, I have read she is better as a Thief but I’m wondering when should I switch her back. Any input?