Do I need serializable for this query?

I have a table that keeps running totals. It is possible that this
would get called twice simultaneously for the same UserID. Do I need to
put this in a serializable transaction? Since this gets called often,
could it be a performance problem if I make it serializable?

CREATE FUNCTION UpdateTotals(IN UserID int,
IN AddToCount int,
IN AddToSize bigint)
RETURNS void AS $$
BEGIN
UPDATE
Totals
SET
TotalSize = TotalSize + $2,
TotalCount = TotalCount + $3
WHERE
UserID = $1;
END IF;
END
$$ LANGUAGE 'plpgsql' VOLATILE;