Whenever I have a survey to read for any house, the sides rarely match to the inch, so what does measuring to the inch prove? I've always rounded to the half foot so that the sides of a home match up.
In Maryland, the tax records typically do include two story foyers for each level, and also open great rooms if the home shows two levels on the exterior. For this reason many appraisers in this region did include open areas for each level in order to use the same measuring technique as what shows on public records. For those now using ANSI, they better be adjusting the comps stated GLA as well in order to have a true comparison, which will involve estimating what is on the inside of each comp home. Of course the automated reviewers and personal reviewers will be screaming bloody murder when they see comp GLA's that do not match tax records.
For those that don't take this into account on their comps get ready for lawsuits and discrimination complaints for severely undervaluing the subject by using different measuring standards for the comps than they do the subject.
In this region at least, it will be a real clusterf*ck.