Six minute markers are what I teach, and never had a problem with 'em, PROVIDED they are not recognised as the be all and end all. As for being useless over featureless terrain, that's not correct. It matters not that there isn't anything there every time - it's the continuous plotting ad re-plotting that is important. It matters not that things may change during the flight - (providing it is only the W/V!) And getting the trainee pilot into the groove with respect to good solid nav techniques is all important. Now provided you can use things in conjunction with six minute markers, ie not use them in isolation then there is no reason why they can't be used without overloading the student or getting them hopelessly lost waiting for a planned feature to turn up. If that happens then there are more serious problems afoot than using six minute markers.