About half-way into our 2000 mile tour this year, my square taper BB developed a creak. It was clearly the BB, since after arriving back home, I replaced it and the creak was gone (I also tightened the BB during the tour with no effect). I have had this happen before and although it did not affect the tour, it was annoying. The BB was relatively new. I assume grit worked its way past the seals of one of the cartridge bearings. I removed the seals of the bad bearing and found that the balls and retainer were intact.
I have always thought that an additional external piece of rubber or plastic might help to keep grit out. When replacing the BB, I discovered that a garden hose washer was the right size and has a rectangular cross section, so I place one on either side. Perhaps this will help. Has anyone else added something to the axle to protect the BB bearings?