It is a truism in appellate practice that the respondent/appellee is in the best position; the standards of review and presumptions largely weigh in favor of affirmance, and so winning in the trial court is statistically the best way to win on appeal. In the spirit of making lemonade from lemons, however, one of the benefits and joys of being an appellant is getting to file a reply brief.
In California and federal appellate courts of appeal, reply briefs are nominally optional. In practice, all this means (or at least all it should mean) is that your appeal will not be dismissed for failing to file one. But from the perspective of good advocacy, there is really nothing optional about a reply brief. The opening and responding briefs set the field, and the reply is where the battle can be truly and most effectively joined. Indeed, some appellate judges/justices and clerks report (confess?) beginning their review of a case with the reply for just this reason.