Yes, getting the fabric lined up perfectly is the hardest part. I photocopy in color the fabric, making 6 copies, and then cut it using the triangle size I want to use and play around with the paper parts.
the first photo below are some blocks I put together using photocopies. The second photo is a OBW in progress that I am still looking for the perfect green fabric to use as an inner border. I was cautioned that my fabric had too much cream open space but I think it turned out great and changed a lot of people's minds about needing your fabric to be very tightly designed. Strangely, while the fabric in both of these photos looks very similar, they are different. I guess I am drawn to this combination of floral designs.