Jedinite is on the money. There's no hard and fast rule, regardless of what people say. I've bought many a very high end power cord and found the most noticeable results on the preamp, not on the line conditioner. Just break it in, and use on each one for several days to give yourself enough time to become accustomed to the sound. Then switch. The reason I say "days" is that it takes time to become emotionally connected. When you switch them around, you'll know which way affects you the most (which will usually be the path that leads to the most emotionally communicative rendering of the sound). THAT is your answer.
Where to put my new power cable?
I just recently purchased a Zu Audio Bok power cable. This will be my first after-market power cable, so I'm not sure what to expect. I intend to use it on my front end component section. What I'm curious about is whether I should connect it directly to my Manley Shrimp preamplifier and then go to my Furman power conditioner, or whether I should hook it up to my power conditioner and then to the wall outlet? I know that, ultimately, only experimentation will tell, but I'm curious as to other's experiences...