my $0.02
There are reasons to only put like devices on the same IDE channel. There are even reasons to have 1 device per channel.
1) ATA vs ATAPI - if you put them on the same channel, you force windows to use "compatibility mode" - a generic, sloooowww, built-in driver - very much like the video driver loaded in safe mode.
2) your IDE controller may not even initialize in some configurations (1 channel with only a slave connected is an example).
3) if you are intending to use one of the fault tolerant RAID levels (1, 5, 10 or 0+1), you should have 1 device per channel - otherwise you will likely not be able to recover from a drive failure. For RAID 0 it doesn't matter, since if a drive fails the data is lost anyway. I've never heard of anyone actually using any of the other RAID levels
4) depending on the IDE controller & OS, mixing speeds could force the higher speed drive to use the lower speed.
5) if you typically access all drives simultaneously, you may benefit from having 1 device per channel. IMHO you would only need this on a server.
Some optical drives use the ATA standard, others use ATAPI, hence the general precaution of putting optical drives on a separate channel from HDDs. So, bottom line is, yes, it can make a difference, but depending on the capabilities of your controller/chipset, and the specs of all drives involved YMMV.