Like tino said, it is really important to see how your store operates from the customers point of view. While you can do a level of testing by using the check/money order or gift certificate options, it still doesn't really test the whole process - from start to finish - that the customer will experience if they pay by credit card or paypal, etc.
One option is to create a product for $1, say, and then run through a couple of test purchases. I figure that I get the proceeds in the end anyway, a few small purchases are well worth the money to have the peace of mind that if I have customers sitting in front of my store, credit card in hand, that they are not going to be faced with an error and end up doing their shopping elsewhere.
Just don't forget to delete your test product after you're finished testing.