For the checkout process I've always just added an item to the cart as its usually pretty quick to do. You should also be making sure those pages look well with multiple and different types of products in the cart.
You'll also want to test out scenarios where the customer is logged in. Creating an account and saving address information will help in that scenario.
The page that is usually tricky is the checkout success page. It's possible to prevent the session from being cleared and allowing the page to be refreshed:
Modify (temporarily) app/code/core/Mage/Checkout/controllers/OnepageController.php
successAction method and comment out the following line: