Yes, you should. From a purely data-integrity perspective, your data gets messed up if you leave pages off of tracking.
Imagine if Pages A and D both have GA tags on them, and B and C are your private, untagged pages.
If someone Lands on Page A, clicks to Page B, buys on B, takes them to C (receipt page), and then they go to D, Google Analytics views that as two separate visits, the second one referred by your own site. This is obviously not what happened, but your data will say that is what did happen.
On the privacy front, Google's Terms of Service stipulate that they do not use your data; indeed, they have too much of it to ever make use of it even if they were that evil.
Further, if your eCommerce takes place on a secure server (https), then Google Analytics tracking is done via secure methods as well, so none of your private data is publicly 'listenable' to malicious third parties.
Google does not have access to anything you don't send it, so as long as you're not passing credit card numbers (which would be a violation of the Terms of Service, and a horrendously bad idea) or personally identifiable information (names, addresses, etc), you can trust that the data is treated safely and securely. Google views Analytics as a lure to sell AdWords more effectively, and to that end, have an interest in keeping data secure to maintain their core business model.
From the Google Analytics Terms of Service:
- INFORMATION RIGHTS AND PUBLICITY . Google and its wholly owned
subsidiaries may retain and use,
subject to the terms of its Privacy
Policy (located at
http://www.google.com/privacy.html ,
or such other URL as Google may
provide from time to time),
information collected in Your use of
the Service. Google will not share
information associated with You or
your Site with any third parties
unless Google (i) has Your consent;
(ii) concludes that it is required by
law or has a good faith belief that
access, preservation or disclosure of
such information is reasonably
necessary to protect the rights,
property or safety of Google, its
users or the public; or (iii) provides
such information in certain limited
circumstances to third parties to
carry out tasks on Google's behalf
(e.g., billing or data storage) with
strict restrictions that prevent the
data from being used or shared except
as directed by Google . When this is
done, it is subject to agreements that
oblige those parties to process such
information only on Google's
instructions and in compliance with
this Agreement and appropriate
confidentiality and security measures.confidentiality and security measures.