I've found that just sanding the interiors smooth, with the grain, yields pretty good sounding results. If you have decent bearing edges to start with, maybe lightly sanding them to smooth the grain, and lightly waxing them, I think the drum's sound is already there. I've never finished a MIJ drum where I had gone through this process and felt like I needed to get back in there with paint, lacquer, etc to try to make it sound better, deeper, louder...
Now, the one argument that I can get behind on this subject is John's philosophy of using a lacquer to seal the grain for the purpose of strengthening and preserving the wood of the thinner 3 ply shells. While I do think that this may be a good idea, I think it has more to do with restoration and preservation than with actually improving a drum's sound.
If you want to paint the interiors white or gray, or throw on a few coats of lacquer or stain, I'm not gonna tell you your business, but please, for god's sake, dont use polyurethane. That stuff will never come off, it will be a permanent modification and I cant imagine that it will make the drum sound any better, but very possibly worse.