I have not found it to be innaccurate, But with web pages (not Antenna Helper) I have had them give me the vector to the STUDIO instead of the Transmitter.
Now in some cases (IE: Channels 2 and 7 in Detroit) the difference is measured with a yard stick, and not many of them (like 50 yards) but in other cases (Channel 41 Battle Creek or Chan 3 Kalamazoo.. Closer to 40 miles.
But then on all the above I have been to the transmitter sites (Well close enough, in some cases in the site parking lot).
From what I can tell, Antenna Helper is giving me Tower sites, not studio sites.