RuthanneC, Blocks are helpful if you want to apply the same display logic to a large group of questions. You can set that up in survey flow instead of applying it over and over again to the individual questions. It is also helpful if you want to randomize a group of questions (but be warned page breaks don't work with question randomization). Another funny thing to know about blocks is that you can't use the back button to go from one block back to the former block.
So if you don't have complex logic and want your respondent to go back from any question to the previous page of questions you should be using page breaks instead of blocks. But if you want to apply complex logic then blocks are good. You can still put all your questions in one block and randomize the order of some questions, you have to use block randomization and instead of clicking one button to randomize a block (if you used that for your organization structure instead of page breaks) you'd have to choose the advanced randomization feature to indicate which questions to not randomize and which ones to.
Some other advantages of blocks that are helpful in testing:
1) you can randomize the order in which blocks appear in the survey using survey flow
2) if you are unsure of your question flow and anticipate you might have to move sets of questions around, Survey Flow makes it easy to do so. Whereas using one block would force you to move the questions within survey editor which I just find harder because of a longer view.

Thank you, that clears it up for me.