Reading novels as research

I've read a lot of differing opinions on whether or not a writer should read similar novels to what they want to write.  Some say yes, because it helps you to gather information for your book and to learn about the market, while others say no, because the risk of "borrowing" tone or style from other writers is too high.  Some say only read them before you start writing, and stop once you begin working on your novel.

I've been doing a little of everything.  I read fairly quickly, so I'm constantly reading something else, and something else, and something else.  A few of those novels have been similar in some theme or another to what I'm working on, but not all that many.  When you consider that I also haven't been working on my novel consistently this entire time, I suppose the risk of "borrowing" isn't too high.  But I have found that the similar ones have all contributed something to my understanding of my setting (1920s Chicago).

One book in particular I found helpful, not only because of the book itself but also because of the author's website.  I recently read the second in a series of YA books about the 1920s, and at the end I saw a reference to the website.  I checked it out, and found this little gem: a glossary of 1920s slang.  I'd been compiling a list on my own, but finding this greatly sped up the process.

What about you?  If you write fiction, do you find that reading similar books to what you are writing helps or hurts the creative process?