Fore/back ground can be kept in focus if it relates to the subject and helps lead the eye. (converging/diverging lines) Keep it out of focus if it is "clutter" (distracting). Sometimes the background just out of focus so not be distracting, but in focus enough to tell what it is is a nice effect and helps viewer to relate (Kid on a swing with slightly out of focus kids playing in the backgroud)
Fill the frame (viewfinder) and you'll have less to crop in the darkroom. Don't try to include everything your eye sees. Just doesn't work.(for me anyway).
Bushes, trees, fences, lightpoles can often be used to "frame" the subject. Effective for land/cityscapes.
Most importantly -for every rule, there is an exception- except this one...