Hi Faye, Phyllis and Scott are right - if it's listed on the MLS, the agent has the opportunity to check it's pinned correctly on the map and move it if necessary. You could always do a Google Earth search for the address and see the bird's eye or street view of the property. Part of your due diligence should also be taking a visit to the town office and pulling the property file, seeing the subject property lot map etc. Good luck in your property search! :)
If the listing agent changes the location of the house on the map when they enter the listing, then it will be correct. for the listing and all the places the listing feeds out to. The default maps on Google and Bing are sometimes not correct as they stand.