You won't be able to put a machine at a show on layaway. If you purchase a show floor model or classroom model you will get a really good price. AND since you are buying from an authorized dealer you can get the classes at your local authorized dealer.
Also consider that with the "big three" coming out with new models - there are two avenues to consider - 1. older, discontinued models are going to be marked way down; 2. people who have to have the latest and greatest are going to trade in models that probably have more features than you were looking for at very good prices.
Really, the best time to buy is when you have saved up enough cash and don't pay layaway charges or interest fees!