"He who marches out of step hears another drum." ~ Ken Kesey
We print the black on the under base to keep all of the ink textures as close as possible, especially after the customer has done the first wash and dry, the 1 hit of black will tend to fibrillate if its direct and give a different texture, but it really always boils down to each design and what the customer is looking to achieve as an after washed product. I see shirts in retail all the time where the printers would not print the black outlines ect on the under base and after the first wash really loses the quality of the artwork's look. It really comes down to what you want to represent your company as final products too. We took different printer's advice, and wash tested various prints using different methods to find that keeping the ink textures as close as possible will keep a long lasting print that keeps the customer happy.