My recommendation would be to install on all servers. If the server has processes that Dynatrace can monitor at a code level (eg. Java, Tomcat, Apache, .NET etc.) then use fullstack mode to get that code level tracing visibility.
If the server doesn't have processes Dynatrace can trace at a code level (eg. database servers, email servers etc.) then toggle the agent into "infrastructure only" mode.
If you get this wrong, you can always adjust the monitoring mode after installation.