What you are talking about is
relative hights from one part of the battlefield to another, not
absolute hights. You do not need to build scale mountain ranges in the living room (unless you are modeling Mount Jackson 'Close Encounter' style ÂÂ

). In the sixties and early seventies people used sand tables to create the desired contours which is the best approach for providing detail, but totally impractical. I am definitely with the others here that we really only need to represent the different relative heights rather than modeling them.
This boils down to whether anything can climb the slope or not and what effect that has. So now we are describing the slope of hills as 'Gentle', 'Steep' and 'Cliff', where Gentle slopes have no effect, Cliffs are impassable and Steep slopes both slow movement and confer an advantage to one side in an assault. (Actually you really need to classify slopes differently for infantry and vehicles). I guess you can use contoured hills and measure the distance between contours to determine the slope, or use slightly different shades of colour.
Taking the question to its ultimate conclusion; How realistic do you like to make the battlefield? (And here I am refering to geographical realism). Often E:A battlefields are represented by a random set of features scattered across the table rather than a coherent representation of hills valleys etc (the 'big hills' mentioned earlier). Does anyone here use modular terrain, or try setting up historical battlefields?