N-600 is super useful in the future. Your step daughter may be asked for certificate 30 years later applying for a job at US government, renewing passport or trying to get a benefit from SSA. The more time passes, the more difficult it is for her to reprove how she became a citizen, especially when neither you or your wife are still around. Having the certificate will make her life so much easier. You can wait a few years before getting it, but don't ignore it, that's my advice.