Time for another round of "when is it OK to omit the multiplication symbol?"

I think for powers of numbers, it's fine to omit, e.g.

2²3² = 36.

What about subscripts, for the number base, e.g.

12₅23₅ = 331₅,

or do I need a multiplication symbol:

12₅ × 23₅ = 331₅.

Or, less esoterically, when I've got a subscripted variable followed by a number, e.g.

x₂4

(I will not accept "put the 4 first" as an answer)