Depending on how your box is built, using the flush trim bit may not work on the front or rear of the cabinet because there is nothing for the bearing to ride on.
Best,
Ed
Yes, you'd have to maybe make a guide for the bearing to ride on while trimming the front/rear edges. Even with a guide, you'll need to be careful at the joint between guide and side panel, if there is even a little seam, it will traslate into a blemish from the flush trim bit.
Don't forget, once you do this you'll have those "fun" seams to deal with. They are much harder to eliminate than people think and even harder not to have them telegraph up through paint down the road.
Best way I know of is epoxy , numerous layers and lots of block sanding.
jay