A QWERTY-based touch-typing tutor I have used a while back, "Solo on keyboard", suggested alternating thumb usage depending on what hand you've used to press the last key, which seemed like a sensible policy to me; however, Colemak lessons I am taking now on the web-based typing tutor Type Fu suggest using a left thumb on all occasions.
Is there an official policy on that matter and how do you do this yourself?