The worst that can happen when deleting an older Cakewalk DAW after installing a new one is some of the shared components used by all the Cakewalk DAWs will be deleted. Re-installing the new one again is the fix.
It is fairly common to leave old version installed. There is so much overlap between versions little is gained be deleting old ones. About half the disk space used in SONAR Producer are samples and loops. For example, deleting an old version of SONAR frees up about 200MB of disk space, the space taken up by the actual program and configuration files. One of the advantages of having older versions installed is the ability to open projects in the version which created them. Generally Cakewalk DAWs are backward compatible; however, having the version which created a project available has proven useful, especially when moving from 32bit to 64bit DAWs. In this case, it did not appear that dealing with a large body of old projects was an issue. That is why I mentioned deleting MC6 in the first place.
FWIW, the reference guide is a snapshot of the original help files in pdf format. It is not maintained as updates are made to SONAR. The most current version of the help files are the web-based help on the Cakewalk servers. The chm version installed on the PC is a snapshot of the web-based help made around the time of the update.