If the information in a parenthetical pause relates closely to the sentence, enclose it in commas. Otherwise, use the dash or parentheses:
The committee's decision, to say the least, sparked considerable controversy.
The members of the committee were, generally speaking, experienced in the biological sciences.
The chairwoman—known for her candor in contractor scandals—submitted her report.
The report on a tax cut (it had already been leaked to the press) condemned the complexity of the tax code.
The report failed, however, to uncover the true nature of the problem.
The agency, consequently, needs additional funds in the next fiscal year.
Exception to the Rule
Again, there's an exception. Sometimes single adverbs produce no pause and therefore require no commas. Watch for adverbs directly modifying an adjacent verb or adjective or other adverb:
The report was indeed deficient. I therefore urge you to rewrite it. It will perhaps influence the president himself.