That's fine, that works. If you want more of a 'terrain' kinda look, rather tahn flatness, check out these links:
http://qeradiant.com/manual/Terrain_Manual/start.html
http://tarot.telefragged.com/gensurf/
I used Gensurf in one of my maps, it's pretty cool (though it requires some practice). Then you can just tilt the terrain mesh or something to create a slope.
The only problem with terrain meshes is they produce a LOT of polygons. So, outside performance may suffer. Something you could do alternately is make a flat ground, surround it with something (like a wall or whatever) and make a sky texture that looks like mountains. You'll get a similar effect without the performance hit.