In a range with unused hosts, is the standard to set the rDNS to a generic default one within the network (such as the IP address dot the company's domain name) or is rDNS not meant to be set for an unused host until manually done otherwise as soon as an IP address is used?
Additionally, is there an RFC which supports any answer?
Pre-assigning reverse DNS to IP's is an acceptable practice. Its use is strongly hinted at in RFC2317 with the words "For each size-256 chunk split up using this method, there is a need to install close to 256 CNAME records in the parent zone. Some people might view this as ugly; we will not argue that particular point. It is however quite easy to automatically generate the CNAME resource records in the parent zone once and for all, if the way the address space is partitioned is known."
