I would be interested in what unit you are using and what kind of area you live in. I use a Garmin GPS 60C and a Garmin Oregon 300. It is rare I can get them both to agree exactly on coordinates. As far as being 'a bit naive', there are probably many things I am 'naive' about; but, I'm not so 'naive' as to believe that even with the newer GPSr units coming out that anyone is always going to be 'spot on' in the coordinates they give for every single cache they place. These units are not built to military precsion; that's why most good units have an accuracy function that allows you to determine just how accurate your unit is at any given time. I've had times when I could get the accuracy to be no better than 40 ft and decided to pass on placing a cache in that location for that reason. But, if you have a unit that can 'guarantee' you will always be 'spot on' with your coordinates I would like to get my hands on one of those.
BTW, you should always double, triple or quadruple check your coordinates when hiding a cache.
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I am new to geocaching but here are my observations so far. Most caches are placed under trees. I don't think that any amount of averaging will compensate for capturing coordinates in areas where there are natural obstructions to the GPS signal. This is how I think the coordinates should be determined. After deciding on a location, get in an area where there is open sky above and your GPS compass reads a pure north or south direction to the cache about 20 feet away. From here record the east/west coordinates. Likewise move away from the cache into an open area in the east or west direction and record the north/south coordinates. Do this several times and on different days and average. You will be very close. When checking, make sure your GPS points to the cache from every open area. It might also help to calibrate your compass immediately before recording coordinates.
When I am looking for a cache, I use my compass to get a line of sight from 20 feet away with an open sky overhead. I try to intersect the projected lines from a couple of directions, but always consider that one set of coordinates could be off, so I'll try an east/west or north/south only, and use the GPS distance to cache.
