BILD has rather close ties to Bayern so the stories concerning them carry some reliabilty, but calling them the paper with the best sources in German football overall is far from the truth. Thing is, that in the German sports press the accuracy of the respective papers vary A LOT from club to club. There is no way in hell that BILD has the best sources in terms of Dortmund for example. Röckenhaus ("Süddeutsche" and by far best source), Dersch ("Ruhrnachrichten") and Hennecke ("Kicker") have all way more credibility here. The BILD wrote so much rubbish in terms of Dortmund so far it is not even funny. My personal favourite is still how they wrote Hummels to Barcelona 2012 one day before he signed a new five year contract without clause.
You are correct, that no German paper ever wrote something serious about Hummels and United, though. This was the main reason why every German poster on here was convinced that the deal was never on.
Overall, the most credible paper in Germany is still the "Kicker" because they have at least decent ties to every club.