After some searching, I can't find any references in Mac documentation, but on the PC end, certain PCI slots are mapped to certain hardware interrupts. Since there are only six interrupts, and all of the PCI slots have to share them, along with onboard devices, it was usually a good idea to put PCI cards so that they used an interrupt that was not already in use. (In some server motherboards I dealt with, this was impossible. There were enough onboard devices to use every single interrupt, so if you plugged in a card, you were immediately sharing.)

If you HAD to share an interrupt, it was a good idea to either always have 'low-bandwidth' devices sharing, with 'high bandwidth' devices alone. (So SCSI on one, Gigabit Ethernet on a second, USB 1.1 sharing with sound on a third.)

This has nothing to do with bus mastering, and everything to do with interrupts. (Which is a legacy of the PC's old fashioned IRQs, which is why I'm not certain how the Mac handles it.)