I can accomplish what I need by simply clicking on the session. The first time I need to select Raw for the request and response, a single click on any of the subsequent sessions leaves them selected. Good enough.
As EricLaw points out, a true solution to this is possible and very simple:
You can set the default request and response inspectors as follows: Click Rules> Configure Rules and scroll down to the OnBoot handler and uncomment the lines that set the default inspectors. Then change the two instances of the text "HEADERS" to "RAW".
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