I beg to differ, sambob... someone struggling with their rhythm might have a problem picking 16ths at 140 b.p.m., which is over 9 notes per second! I think practicing a bit slower at first would help more. I think practicing maybe eighths at 120 and working on nailing the note precisely on the beat would be more beneficial at first. But the idea f switching between odd groupings is a good idea for a more advanced player.