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In animation everyone tends to work slightly differently.  I am not saying that this is the only way to work but I tend to work in the following curve flow in my process.

Pass 1 - Stepped: Great for setting up thumbnail style key poses.
Pass 2 - Linear: Good for setting initial time. Action too fast = Keys too close together. Action too slow = Keys to far apart.
Pass 3 - Auto: Best of both worlds Spline / Plateau
Pass 4 -  Fixed, Plateau, Flat, and or Spline. Very specific curve type for specific reasons.

Maya Curve Types

1. Stepped
2. Stepped Next 
3. Linear
4. Spline
5. Flat
6. Plateau
7. Clamped
8. Fixed
9. Auto (New 2012)

Found on toolbar

    Stepped - Constant value until there is a new value.  A good use of the Stepped Curve is with a proxy camera to establish camera cuts.  To avoid motion blur issues you should replace the proxy camera positions with separate render cameras.

Tip & Trick:

1. Create or Import Render Cam
2. Parent constrain the render camera to the proxy.
3. Delete the constraint.
4. Advance to the next camera placement.
5. Repeat 1 — 5 for all camera positions.

    Linear - Constant rate of change.  This curve type is great to use while trying to establish the initial timing on an animation.  If the action happens to quickly then the keys need to be spread further apart.  If the action happens to slowly then the key need to be brought closer together.

    Auto - An Auto tangent type will adjust the tangency of a slope as to flatten out as it get closer to the value of the key adjacent to it.  THis is a nice feature and seems to be based off of the AutoTangent Script.  THe best of both spline and plateau seem to be at play here. Pitfall: the slope flattens more than needed as the key is adjusted. If you manipulate to the desired angle and then move the key it will hold the desired slope.

    Spline - Equal slope going in and out to a keyframe.  This will give nice smooth motion; however, it will create overshoots that are unwanted.

    Flat - A flat curve type simply sets the tangent angle to be a zero slope.  Most common pitfall I see students doing is selecting the whole curve my mistake and then converting to flat.  The will cause a slow and go type of effect like a car with an acceleration problem!

    Plateau - A Plateau curve type will eliminate over shoots as long as the key frame values are within a tolerance range.  This range is greater than the clamped function. Pitfall: Converting the whole curve. Make sure you only use it where necessary or it can interfere with ease in situations that you have already set up.

    Clamped -  A clamped curve type was most commonly used to eliminate overshoots on foot and leg movements. Basically we two keys are with a small value difference from one another that different on slope is clamped off.  The end result is much like flat and plateau but the tolerance range is much smaller.  Pitfall:  is in not zooming in on your curves to make sure that the values of the keys are equal.

Found in Menu Set

Fixed - A fixed curve type simply fixes the slope of the curve to a specific value.  If you move the value or time of the keyframe the slope will remain the same.  Note: Anytime you break a tangent it automatically converts to a fixed tangent type.  If you are curious as to what the tangent type is for your keys:

1. Select some keyframes
2. In the Graph Editor navigate to Display -> Spreadsheet.

In Maya 2012 the tangent type button at the top of the graph editor activates when a single keyframe is selected.

Stepped Next -  Stepped Next curve is used with FBIK.  The difference between the Stepped and Stepped Next is that there is no constant value out.  It simply is reversed so that the value does not hold on the out tangent but rather switches to the new value immediately.