I was having a conversation with my dad this afternoon about our current tiling project and the question came up on whether it is better to keep your lateral lines at a fixed grade or a fixed depth (assuming the ground has adequate grade to begin with).

Case in point, we're putting in laterals right now in a field that has plenty of natural slope, but has a lot of small humps and valleys along the way.  The oposing arguments are:

A constant grade would keep the water flowing at the same velocity through the pipe to the main.  However, the line might be too deep in some places to really do much good at getting water off the surface through our tight soils.  

A constant depth following the natural rise and leveling off of the land would allow the water to be drained much more evenly across the field.  Although the water may not flow as well because it would constantly be trying to speed up and slow down due to varying grade of the line.