When should we use sizeof with and without parentheses [duplicate]

Answer 1

[expr.sizeof]/1:

The operand is either an expression, which is an unevaluated operand (Clause 5), or a parenthesized type-id.

Thus the parentheses are required for types only. If you prefer to use parentheses for clarity and consistency (as I do) you can always use them though, as parentheses around an expression form another expression.
The operator precedence of sizeof is not very well known and could cause irritations.

Also, for the sizeof... operator, you always have to use parentheses (another reason for consistency).

Answer 2

When using sizeof with a type, you need parentheses around the type. When using it with an expression, you don't. But you can of course include them in this case as well, and you don't have to worry about operator precedence in such case. With uncommon operators such as this one, fewer people would be sure of the precedence, so clarity certainly helps.

So I'd say it's preferable to use them always.