It's a fine line. You don't want to get a source's hopes up by promising her coverage Big Name Month Mag (or waste your time doing tons of legwork on an article that may never see print), but you also don't want to disappoint an editor by pitching her an interview with a source you can't deliver.
I got into hot water once because I got my editor excited about a story, then couldn't get the source to return my emails. This was a profile piece, so we had to scrap it completely, but at least I'd worked with the editor on several other stories, so she knew I wasn't a total flake. Still, I did lose some face with that one and learned a tough lesson.
I've come to the conclusion that if the article is one where you could slot in any number of sources, then you can pitch it by saying "potential sources include Jane Doe, author of... ." The phrase "potential sources" is key because it shows the editor that you're given it some thought, but you're flexible enough to consider alternate sources.
Jane might have written a killer book on DIY fashion or interracial dating, but chances are, you can find another expert if you have to. Since the article does not hinge on her opinions about a highly specific topic, you can wait until you have an official assignment before you email her publicist (some will happily forward you background information and others will give you the cold shoulder at this stage).
If, on the other hand, you hope to pitch a profile, then you should make sure that your source is receptive before you get an editor involved. Recently I had a slew of potential sources refuse interviews for reasons I never would have anticipated (fortunately, I didn't pitch the article and it didn't have super-specific requirements, so I was in the clear). You might assume that Jane Doe would be thrilled to do an interview and promote her book, but she might have her own reasons for not wanting her profile to appear in a certain publication. Or maybe she's so busy doing interviews with The New Yorker and Rolling Stone that she doesn't want to spend her precious time on a mid-sized magazine. If it's an article with a variety of sources, you can just find another one, but if it's a profile, well, you're kind of screwed.
During the query stage, it's important to emphasize to your source that you are hoping to write an article about her, but it's not a sure bet. If you can get away with saying "I'd like to pitch this to women's magazines" or "I'm going to shop this idea around to some travel websites" instead of dropping specific names like Cosmo or Concierge.com, so much the better. You don't want them to get their heart set on a certain publication, but some people will press you for details. This is something I still struggle with!
Another strategy is to reuse sources you've used in the past. It's (generally) a safe bet that if someone was willing to talk to you in the past, they will probably be receptive to doing another interview in the future. I wrote a profile about a young businessowner who was so excited to see her story published online that I had a hunch she'd be happy to do it over again for a feature article in a different pub. I was right. Plus, you can often include unused bits from your original interview to beef up your new query.
What do you think? Do you usually conduct a pre-interview when you're writing a query? How do you explain this to sources? Any other thoughts on the topic?