In this case, I find the parenthesis to be helpful in reading the query. Without the parenthesis and outer SELECT statement, it might not be obvious to a reader whether the ORDER BY applied to the result of the UNION operation or whether it applied to the last SELECT statement in the UNION.
Under the covers, Oracle will treat both queries identically, so whichever formulation is easier to read ought to be chosen. To my eye, the outer SELECT makes the query more obvious, but that is purely a matter of taste.
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