General rule to be followed in order of most to least demanding drive: isolate/master/slave; with preference for placing on seperate channels those drives with the most transfers betwixt them.
So with a single HDD, DVD writer, and CD writer put the HDD on IDE1 as master (obvious), DVD writer on IDE2 as master and CD writer on IDE2 as slave. Only if those two optical drives were going to be used primarily for copying together should the CD writer instead be put on IDE1 as slave.
Likewise with introducing a second HDD it would depend upon whether removable (in which case limiting it to slave on either channel) and the primary way it was to be used in relation to the other drives.
Also be sure to put master at end of ribbon cable and slave in the middle.