Hmmz... yes we also never had inconsistant serial numbers here. But I always use the default which is something like hostmaster.domain.com so for admin's domain it's like hostmaster.admindomain.com and we use the nameservers there too.
Then in /etc/aliases we point root mail to the mail address where we receive reports from postmaster and hostmaster and whatever is present in the /etc/aliases file.
I wouldn't even worry about it to be for real. Especially if you're no DNS expert (which do exist) I would leave things as they are default, less chance to mess up something by accident.
Because if there is an update and you have a customized file, then be aware this will not be updated, you have to do that manually. Which is why imho it's best to leave things default if it's not really required to change them for some reason.
If you don't make mistakes by customizing, things will ofcourse keep working as should be.
But if it's for one domain, if I'm not mistaken, you have to change the db file for it. Then still issue the bind command to increase serial number so changes are passed to the slave.
However, I did not test and don't know, if on any DNS update of the domain, the .db file will get it's original SOA address again. Because you can't customise those files in a /custom directly like configs, as they can change more often.
So well... it's a choice, but I would just change the root address in the /etc/aliases file, which is wise to do anyway.
Like change this default:
#root: marc
to
root: [email protected]
then restart Exim and you're done.
And then you don't need to worry about editting bind db files anymore.