You can easily create custom viewport views while in crop mode with the Set 3D View command.
1) Double click on the viewport.
2) Select Edit Crop and click OK.
3) Select View > Set 3D View. From the dialog box, you can specify a viewing height, angle, and perspective.
While in crop mode, you can also use the numeric keypad to change to standard views as you can on design layers.
Edited by JimW