[jborean93]

That's the realm side which should be upper case. The comment reference was for hostname themselves which I've always just done as lower case and have never seen a reason to make it upper case. The krb5.conf has a [domain_realm] section which can map a DNS name/suffix to the actual realm

    [domain_realm]
    .domain.com = DOMAIN.COM
    domain.com = DOMAIN.COM

[woleium]

iirc, on the windows side, workgroups had to be upper case, so initially the krb realm was set to the workgroup name. dns came later