Unless ordered, a reply to an answer is not necessary pursuant to the Federal Rules of Civil Procedure (here is a great practice guide you’ll want to keep in your office). Specifically, Rule 7(a) establishes that plaintiffs may not file a reply to an answer except in specific circumstances. In those circumstances, a reply to an answer is due twenty-one (21) days after the order for a reply is served. Fed. R. Civ. P. 12(a)(1)(C).
This general rule for civil litigation is contrasted with the rule for state inmates seeking habeas relief, which allows a reply by a petitioner. See Rule 5(e) of the Rules Governing § 2254 Cases (“The petitioner may submit a reply to the respondent’s answer or other pleading within a time fixed by the judge.”).