It depends on the design of the diesel particulate filter (DPF). Some TDIs have a system which supplies raw fuel to the DPF during the regeneration phase by passing it through the combustion chamber. When this happens, uncombusted fuel can be scraped off the cylinder walls by the rings and pulled down into the oil sump.
Someone else can speak up regarding the Q5's DPF technology; it may not use the described method, so may not be prone to fuel contamination. But the owner's manual should specify what limits are placed on the biodiesel concentration.
A low biodiesel concentration, for instance B02 or B05, is beneficial as it provides lubrication to the moving parts in the injection system. B10 may still be okay -- let's hope so, since in some states that's all that's available. But owners of TDIs with a DPF should probably avoid biodiesel blends greater than 10% (B10). In a pinch, of course, any biodiesel blend can be used, as long as it's high-quality fuel meeting the relevant standards.