A short answer to the immediate question: don't focus too closely. Focusing too far away will have relatively minor effects on sharpness of near-field objects, but focusing too closely can seriously soften the rendering of distant objects.
There is a limit to how blurry close objects can become if focus is too far away. There is no limit to how blurry distant objects can become if focus is too close — well, in practice it's limited by the minimum focus distance of the particular lens, but by then it's usually a disaster anyway.
For common high-DoF landscape photography, the "1/3 way into the scene" rule works pretty well. That'll pretty much assure that you're focused far enough away — provided you're stopping your lens down. It's instructive to do the geometry. Also to read some of Harold M. Merklinger's writings on the topic.
In the end, though, the best solution is to go out and do some test photos with your camera and lens, at various f-stops, various focal lengths, and various focus distances. Theoretical hand-waving is only a guide to what experiments might be worth trying. Unless you're talking about string theory.
The open-source LightZone Project: http://lightzoneproject.org/