Jeff, you didn't mention the dimensions. If the block is wide enough to yeild a good 5"-5.5" long blank, a true 90* cross-cut would look fine. If the blank is not wide enough, I would certainly do a bias cut (30-75*) to get whatever lenght is appropriate.
Another consideration is the grain. I have done some woods with a 90* cross cut that, once I turned the pen, it looked just like a straight-grain pen--- the problem was that the endgrain did not have a strong "bend" or curl in the grain-- it was only faint. Your block seems to have a moderate "bend". In other words, the annular rings in the end grain have a nice tight arc if it is closer to the pith of the tree. If the block came from wood further away from the pith of the tree (especially a big tree), crosscutting may not yeild a grain much other than "straight" on our pens....thus making the pen harder to turn but not nicer.
Since yours has a moderate arc, and not a very tight arc, you might consider cutting them on a bias--- instead of a straight 90* crosscut, try cutting them at a 70* or a 60* angle (a good miter saw to make the first cut is very helpful, and you can bandsaw the rest to save wood). Thus you get a very interesting "loop" look in the pen as well as something a touch easier to turn than a solid 90* crosscut.
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