Does the Data Level matter for an 0x4b polygon? I suspect it doesn't, but if anybody's had problems with it, I'd appreciate your comments.
I have never added a 0x4b background polygon. It may behave as other polygons and display at the bit level specified and more-zooned-in; or Garmin might have it 'hard-wired' to something.
I would make a small test file and see what happens with various values.
I always use a 0x4b polygon because I don't like the default tan background that the Nuvi and Oregon use. I believe the default is a rather putrid yellow on the 60csx also. I also find it very useful to have the background when I export gridded files from GlobalMapper. If I want to add another layer of data, I copy the background polygon from the tile, paste it into the target file, then export using the option to crop to the selected polygon. This helps me keep my maps organized by smaller tiles while keeping larger basemaps of things like roads, landcover and hydro.
When creating custom types, I always set 0x4b to be drawn first, so that everything else will appear on top of it. I have never changed the bit level for these polygons and always just use the default.