Just to be clear, parentheses were never ever used in the original Greek or Hebrew. Where they occur in our English versions, they have been inserted by the translators in order to clarify meaning.
The verses you cited, 1 Kings chapter 21 verses 25,26 are only in parentheses in modern versions such as the NIV and ESV. Neither the King James Version nor the modern New King James Version have them in this passage. They are also not present in "word for word" translations such as Rotherham and Young's Literal Translation.
The bottom line is that the addition of parentheses to clarify the text is a relatively modern thing and is purely at the discretion of the translator. Knowing that, if we think that the addition of a parenthesis might not be correct, we should compare the text with other versions. Usually, though, the addition of parentheses is a very helpful device to clarify meaning.
I hope you have found this helpful.