I feel your pain, as I am also about your height. Most of my cessna time is in the C172 and C182, with only about an hour in a C150.
While you don't have to have the same sight picture as your CFI to be able to fly the plane, it makes training and learning a lot easier if you are both seeing the same thing. When learning pitch attitudes, it is much easier to tell them to set the horizon on a specific point (the dash, or the top of the compass, etc). Quite often, us CFI's might get really used to using these references, and if you aren't seeing the same sight picture, it could be difficult for you to figure out what he wants you to do. There are a lot of students who have trouble with the flare on landing because the instructor is telling them to fly a certain sight picture/pitch attitude, but their seat position is wrong, so they see something different.
Perhaps the most important thing is that your seat height/position is consistent. Any change in seat height/position will also change your sight picture, and the same pitch attitudes you used before will no longer work.
At my flight school, there are lots of instructors and students, and the students are here from Private up through Commercial and CFI, and over the course of their training will fly with multiple instructors. For safety sake and ease of training and transition between CFI's, we are big on standardization of procedures. A lot of time is spent early on helping students find the proper sight picture, and it helps a lot in the long run.
If you ever get to fly any jets, you will find that in a lot of them, there are devices in the cockpit that help you align your seat so that both pilots have the same eye position and sight picture. Kind of nifty little devices, but they wouldn't be there if the manufacturer didn't think that consistent sight picture was important.
Being about 5'5" also, I have never found that I needed a cushion, nor have I ever used one, even when I flew a C150. The C172/C182 is pretty generous with seat height and position adjustments. If you cannot get the seat into the right height/position with the adjustments alone, you should look at the way you are sitting. You should be comfortable, with your back against the back of the chair, not sitting straight up with your back off the chair, or, even worse, leaning forward slightly. Not only does the latter mess up your pitch attitudes, but you can't reach as far with your legs.
Stephen St. John
CFI-A, CFI-I, COMM ASEL/AMEL
Prescott, AZ, USA