What is the final output destination of the newsletter? When you say "printed" do you mean on an internal inkjet / laser copier or do you mean that it will be printed by a commercial printer (web press or sheetfed)?
I would strongly suggest that you output to PDF as you will produce the highest quality output with this method and PDFs are somewhat unchangeable. (I say "somewhat" because it is possible for a customer to make changes to a PDF that does not have any security settings if they have Acrobat Professional or the Enfocus Pitstop plugin for Acrobat.) If you have concerns that the customer will actually make significant changes to your document without your permission or use the document in the future without paying you further for your design services as they should, then I would recommend setting some security permissions on the PDF document. This is easy to do and pretty straightforward. You can do it in InDesign when saving the PDF file or you can open the file in Acrobat Professional and change the security (File—>Properties—>Security). You can allow printing, but not editing.
You can, as you suggest, create an EPS (or you could do a PDF) from ID, open in Photoshop, and then save the document as a TIF. This will certainly ensure that no editing will take place. However, you will want to be sure to save your file at 300dpi and I've seen many people recommend saving at 400dpi or higher if you have a lot of text. You run the risk of significantly decreasing print quality, especially of text. You also run the risk of any solid black text/images no longer separating correctly. CS4, 5 and 6 do seem to handle this better than earlier versions, but in my prepress experience it can be hit and miss. In my opinion, PDF output is really your best option in terms of output, but only you know your relationship with the customer.
Thanks for your response, it's very helpful and confirms what I had feared, that my idea to go .eps > .tif runs the risk of having a negative effect on final print quality. Yes, it will be offset but that's where I start to get fuzzy in terms of their capabilities and preferences. In the past I have worked with internal print departments that were as well equipped and professional as a commercial printer (well, almost). But there have been some extremely tedious folks too. What I don't want to do is provide a file that will give them trouble and a chance to blame me for anything since I'm already on the sh*t list for taking the work from the internal design team.
I think I will ask the client to point me in the direction of the person in charge of printing to determine if they're comfortable accepting a PDF. If so, then I can follow your suggestion and save the file with the right security.
Sounds like you've got a good plan. Glad to help - hope it goes well!