The 10mm, 7mm, 5mm, and 3.5mm all have negative field curvature.
The 14mm, 20mm, 30mm, and 40mm all have positive field curvature.
It depends whether that matches your scope as to whether you see a flat field or not.
Here are the astigmatism/FC curve graphs:
https://web.archive.org/web/20150522.../xo-xw/64.html
If the two curves (sagittal and meridional) deviate, that indicates astigmatism. The center of the field is at the bottom and the edge is at the top. The scale is in diopters (the eye can usually accommodate 1, but 2 would be seen as curved).
Ideally, the two curves would not deviate and they would be vertical.
The 30mm and 40mm have been out of production a long time, but are coming back soon--perhaps by the end of the year.