With some sabaki, its feasible to turn and use shuto as a block, but to spin on one leg and be in a position to defend an attack, which could only possibly be seen at the last minute, seems pretty unfeasible.
Personally I use a slight backward movement in Heian Nidan, Kanku Dai, etc when I have to turn 180 degrees into shuto uke, for this reason. I'm not sure how "ok" that is though

It just feels more natural to me.