I think that depends on who you are popping and what you are trying to achieve.
I know that doesn’t sound too helpful, so I’ll try to give you an example. I’m assuming you are talking about buying pop-unders from someone else.
If you have a site that you are trying to brand and establish repeat traffic, send them to your main page.
You can send them straight to the sponsor, but depending on the source for the pops, your sponsor may become your competitor for the traffic if it is a qaulity source.
If you make a seperate page, you can keep tweaking it for better performance and your sponsors don’t know where you are getting the traffic.
Like I said, it really depends on what you want to achieve.