In a residential setting, trees do add value.
In a rural setting, that's often less the case. It really depends on what's around, what's expected (by buyers), and so on. For instance, in many cases someone buying in a rural setting would expect that there'd be trees. That's part of the whole environment. (When I was a kid, our family lived on 63 acres. Only about 5 of that was cleared--around the house down to the road. The trees were natural; it was part of the environment. Undoubtedly it would have been less desirable if all the trees had been cleared.)
On the other hand, if there's some spectacular view that's being blocked by the trees, then judicious clearing of the trees (obeying all local ordinances and regulations) could add value.
Remember, too, that if the trees are cleared, then you'd probably have to maintain the land in some way--anything from a well-tended lawn to a more general mowing every month or so. Trees (in a wooded area) are pretty much maintenance-free. You don't have to worry about the leaves, or falling branches, or even entire trees falling over. It happens, but it's part of the environment.
So what's the bottom line: In many/most cases, I'd say leave the trees.
Hope that helps.