As you layout TSpline faces and edges you might introduce some waviness. This makes sliding edges and moving edge loops harder than need be and can cause uneven surfaces. One tip to creating great TSplines surfaces is managing the control cage and creating flowing patch topology/edge chains.

To clean up these edge chains we can use a trick to scale the edges in one axis.

To try this:

Start Edit Form.
Double click an edge to select the edge loop, Or select a set of TSpline vertices.

Adjust your view to see the manipulator and select the bar shaped element to scale.

Drag the scale and when the RVEC appears enter 0.

Note that this will move all the edges into alignment but it might not be an exact planarize.

To precisely planarize a set of edges you will need two things.
1. A plane you want to planarize too.
2. You need a set of vertices. Flatten will only work with the TSpline vertices.

To try Flatten

From the modify panel choose pull.

Select a chain of vertices.

select the plane you want

Finish!

Using this technique you can quickly air and straighten edge chains. this works in smooth and box mode. The results should be quickly obvious in the quality of the surfaces.