Neither is strictly necessary or necessarily true. I think a common abacus may serve to substantiate this claim. Recall that the Romans did not know the decimal system. Counting can be done with Notches. Notches can be grouped by 3s, 5s, not only by 10s.
Positional systems arise naturally.
The decimal system is only good provided grouping in 10s is easier than grouping in 5s. Which is not true. Doing so, they miss an important and instructive phase of the work. By looking back at the completed solution, by reconsidering and reexamining the result and the path that led to it, they could consolidate their knowledge and develop their ability to solve problems. A good teacher should understand and impress on his students the view that no problem whatever is completely exhausted.