Been following this with a bit of interest as I've recently placed my first cache. However the guidance is to approach the cache from different directions and take a reading, then look at them all and discount any wild figures etc to get a better 'average'
GPS devices are 'probably' a bit more accurate than smartphones but I would imaging the 'approach from different directions' method would apply to whatever you use to set the co ords.
Anyway, my cache was published on Monday and first find was yesterday so I suspect the co ords are good.