my opinion and experience is:
If you have a single drive at the cable, jumper it as Master
If you add a second drive to the cable, jumper it as Slave (there are some old drives where the Master-drive has to be jumpered explicitly as "Master with Slave present")
Jumper setting is not important if there is only one device in the cable. Both slave and master settings do work, just make sure you dont have 2 slaves or 2 masters in the cable.
