I am going to install another operating system on a partition just beyond MS-DOS 5.0. In the PC-Unix manual it says that some operating systems, "like MS-DOS", write beyond the last cylinder of their partition, so to leave some space between DOS and Unix. I have never heard of this before and sounds wrong to me. But I'm compelled to ask, is it true?

That does sound more than a little odd. I have installed AT&T Unix, Linux Distros and Sun Solaris without leaving a buffer area on the hard drive and have never had any problems. It is a bit of a puzzle. When you create a partition beyond your DOS partition, whether you do it with FDISK or a third party utility like Partition Magic or Ranish, the new partition starts at a point after the Stop Bit on the DOS partition. Unless you are using some kind of very exotic partitioning scheme, I wouldn't worry about it.