Adding internal fillets is a VERY common practice! I would have guessed it is more common than not by how often it is discussed or mentioned in build threads. Forces on the exposed fin, especially something like a hard landing impact, can make the fin want to rotate around the point where it goes though the airframe wall, and that can cause the fin tab to tear loose from the body motor tube. Then you have a much weaker, wobbly fin attachment. Adding an internal fillet helps to bond the fin tab to the motor tube.
To add internal fillets, do not glue the aft centering ring in place when you assemble the motor mount or glue the motor mount into the body tube. Instead, dry fit the aft centering ring, and use it to help position the motor mount straight in the body tube when gluing in the motor mount. But leave the aft CR unglued, and add some tape tabs or something else to allow you to pull it back out ofter the motor-mount glue sets up. Once the motor mount is glued, pull out the aft centering ring, and glue in the fins. Having that aft centering ring out gives you access to the inside of the fin can, and you can add your internal fillets. When I've done this, I've added fillets between every joint that the fin tab touches inside the fin can --- the BT and tab on the inside, the centering ring and the tab, the motor mont and the tab. AFter that, glue in the aft centering ring with glue joints between the aft CR, bottom of the fin tab, body tube and motor tube. Now THATS a bulletproof fin can!
