Yep exactly...CRT burn in isnt some sort of defect, its just the natural aging of the CRTs. The higher the brightness of the guns (which is contrast, white level, not "brightness", black level), the more work theyre during.
If theres a certain area that gets used in a certain way, its going to age in exactly that way. If you watch the same thing all day, every day, anything static on it, station logos, tickers etc, will burn in. If you all do is watch widescreen movies, the black bars are going to burn less than the actual movie area, so when you watch regular tv, itll will be brighter than the "movie area" of the screen.
This of course, is only an issue, if thats ALL you do. If you watch/play all sorts of things, and dont leave games paused overnight etc, no problems at all.
Just dont worry about it.