You can cut them to any angle you want. There is nothing special about 45 vs. 35 or 57.39 degrees. You will get more waste (lower yield) if you do not cut them straight or 90 degrees, but if you have enough material, that isn't an issue. It is my experience that blanks are easier to turn (less tearout) the further you get away from 90 degrees, but the visual effect of the finished pen is different. You will see more chatoyance at 90 degrees, though the striping effect may be more prominent at 45. Depends on the wood. Experiment and have fun.
Joe