Not all were biased, and they were reporting facts not opinions. Also they knew the germans very well and even hired some german horsemen to fight with them. They also had direct contact with the celts (they conquered them), so the facts they reported can be considered accurate enough. If the druids eviscerated people to foretell the future of a battle by the way the man died and how his intestines moved, it isn't the ancient romans' fault 
I mean facts with details, even some druids were named and we know they had direct contact with both populations, what else do you want. You can chose to believe myths and legends without any source if you want, but that's not history.
This belongs in the historical discussion area.
But anyway, Thucydides (the poster here, not the Greek historian) is correct.
Unless you want to tell me that you believe that the Druids had a mystical egg that they captured after it was formed from the 'scum of serpents' who lifted it up so that the druid could catch it before it fell? (Tacitus, in Agricola, and Pliny)
Generally, modern classical historians consider that the ancient sources are not necessarily 100% true.
Don't forget that the study of history was practised in a very different way back then. Mostly it did come from word of mouth - particularly when dealing with areas that the Romans had little experience of.
Also, to talk about the Germans as if they were a nation as they are now is misleading. The Romans had contact with only a few tribes who lived along the Rhine and north of the Danube. Germanic tribes also migrated often, so the Romans had very little idea of who actually lived in Germany or what they did.
Germanic tribes were frequently used as tools in Roman propaganda (Claudius did this most famously).
In short, no you can't rely on the ancient sources for historical accuracy.
