Putting an interface in promiscuous mode permits you to read all the packets that are crossing an ethernet (==diffusion network) segment.
An ethernet segment stops at a switch or router frontier.
So, among the same ethernet segment, you can put ntop where you want, the result will be the same everywhere.
There are techniques to bypass switch filtering, have a look at ettercap or macof. The latter may also work on a router: a router can be transform in a hub if it gets overflowed.
If you don't put the interface in promiscuous, then you can see all the traffic which has destination ip=interface ip; so if you put it on a router you should see a lot of things.
Not sure I was very clear
