When I plant a letterbox, I check GC to see if there is a cache nearby. If so I notify the cache owner of the situation. They are usually cooperative, and we put notes in both our clues that another non-related box is nearby, and don't confuse them.
I usually try to plant my letterbox 200 feet or more from a cache to avoid cross finders. Although one time a cache was planted 20 feet from my letterbox. The cacher was most cooperative, and I have invited him to events to promote caching.
I also check GC about once a year to see if any caches turned up near any of my letterboxes.
The Bird Stamper