Surprisingly enough, most
GIS boundary lines that are shown on county Real Property interactive sites are fairly close to, or close enough for the average person. If the parcel has had a recent boundary survey completed then it will be implemented into the GIS line work and will be very accurate geo-spatialy.
I tell all my clients that I complete a boundary survey on to file it with the county so the line work of your property is correct. Chances are, if the boundary line work is incorrect on the GIS sites (OnX, Huntstand, etc.) then the parcel has never had a boundary survey completed.
Like previously mentioned by @Mennoniteman
, You should be going off from boundary line evidence that you visually see on the ground rather than a line on your phone.