A couple things to keep in mind when placing a cache.
1) Let your GPS settle first before marking the spot. 20 seconds or so should be good, just so it has a solid fix on it's location.
2) Check the accuracy at the time you place the cache. If you have a clear view of the sky, and the satellites are in good position, your accuracy can be as low as 7 or 8 feet. If that's the case, you can mark the cache once and walk away - you're probably not going to get any better coordinates than that.
3) Ideally, you would get readings at different times of the day, since the location of the satellites determines the accuracy, and use the average of those readings.
I usually mark the location, walk about 50' away, go back and mark it again. I do this a couple of times to see if I am getting consistent readings. If the accuracy is 30' (10 meters) or greater, you might consider going back at a different time of day and trying again.
Even if you had absolutely perfect coordinates, it's going to depend on the number and location of the satellites the GPS is receiving when the person goes to look for it. That is why some people report that the coords were dead on, and some say that they were off by 30' or more.
If I find a cache that is 50' or more from the posted coordinates, then I usually take a reading with my GPS and post it on the cache page.
Hope that helps.