Hey,
Making sure you have the right eye height (or at least keeping it constant between flights!) is very important. Without it then as you have said you won't get the right perspective and that can lead to all sorts of trouble, mainly in my experience with landing. There is nothing wrong with using cushions, I know a number of vertically challenged pilots who have their own cushions and use them when needed. The main thing you need to do is find what works for you, so you feel comfortable and can get on with the flying. Good luck with your training!
