There are several schools of thought on this.
One says always put hard drive master and cd-type drive slave (assuming 2 hard drives, a dvd and cdrw for example). Basically the idea here is to keep your CD-ROM on one channel and your CD-RW on the other so you could copy CDs without having to make a hard drive image first... otherwise you risked buffer underruns and stuff like that when doing direct CD to CD-R copying. These days this isn't really much of a problem anymore...
The other is hook all your UDMA devices to the same channel, such as all UDMA66 devices to one channel, and slower ones to another... I've heard that putting a UDMA66 device on the same channel as a UDMA33 or PIO device will slow it down to the speed of the slower device, but I've also heard that this doesn't happen.
For what it's worth, I've got my UDMA66 hard drive as a single drive on the primary IDE, and my UDMA33 CD-RW (10x/4x/32x) as master and my PIO mode 4 CD-ROM (48x) as slave on my secondary IDE channel, and don't have any problems.
<P ID="edit"><FONT SIZE=-1><EM>Edited by Jericho_Narcas on 03/04/01 08:04 PM.</EM></FONT></P>