There are different opinions on this. In general, adding views or fields on forms is OK, but changing fields or views is bad unless you really, really know what you are doing, since the views are used in lots of internal ways.
I inherited a NAB where such changes were made, the result was there were duplicate PEOPLE, USER and Group views, the default data that is usually populated into the address book was either missing or quadrupled, and because someone checked “Do not allow design replace or refresh” in the “changed” design elements, and in order to fix the problem, we had to shut down all the servers and paste a new vanilla copy of the address book on each server and reduce everyone’s access to less than designer so that they didn’t replicate old design changes or duplicate data back into the address book.
If you add views or folders, develop a naming scheme so that you can easily recognize those design elements that are not native to the NAB template. This way, when you upgrade, you’re not faced with the task of trying to figure out which design elements need to be updated.
I’m not sure I would add fields to the person doc, (but that’s me), I would probably create a second copy of the person doc and a view that would display THAT form.
I agree with Ben and Marilyn, but want to add that adding readers fields to person documents is pushing your luck. Consider that missing to place LocalDomainServers in it would probably be the equivalent of removing the user from the directory.
You need to know exactly what you are doing before making such changes.
There are many organizations that don’t allow modification of the NAB design so that there’s no extra maintenance necessary to integrate those changes when the next server template version comes out.