I have not done a right sided or posterior 12-lead yet. When I interpret 12-leads online, I kinda consider posterior and right sided 12-leads not necessary.
A posterior MI, you will see reciprocal changes in the anterior leads. In one case from ems12lead I believe, there was slight ST elevation in V6 because it is closer to the posterior wall. Often, I see early R-wave progression too (opposite of anterior wall MI, where the R-wave progress a little bit later).
For right ventricular involvement, you'll see greater ST elevation in lead III than in lead II. Another thing that might look weird is ST elevation in lead V1, but ST depression in lead V2. Even without doing right sided leads, y'know there is gonna be some right sided involvement in inferior wall MI due to the right coronary artery (RCA) usually being the culprit. Less commonly the left circumflex or very rarely the left anterior descending (LAD) are the culprit, and they will appear differently. When the culprit is the LAD (for inferior wall MI), it presents so weirdly with such wide spread ST elevation that I see people usually screaming "pericarditis". :[
Not sure if this is a bad habit, but that may be why I haven't done right sided or posterior 12-leads in the field yet. Online, I don't feel like either of those have revealed shocking information I didn't get from a regular 12-lead.