$a \cdot(x\cdot\ x)$ or $(a\cdot x)\cdot x?$

Expressions such as
$ax^{2}, axy$
, or
$ax^{3}$
, where
$a$
is a constant, are not unfrequent in computing. There are several ways of parenthesizing them (and therefore, choosing the order of evaluation). Depending on the value of
$a$
, is there a more accurate evaluation order? We discuss this point (with a small digression on spurious underflows and overflows).