I put up my query letter for Thorn of Dentonhill on the Absolute Write message boards, as I was told they would "tear it up and break it down and make it as good as you can get it". Always something I could use more help with, but with all things, you have to be aware of who you're getting advice from. All advice needs to be taken with a grain of salt.
However, this one bit jumped out at me. In my query letter I have the following:
The Thorn of Dentonhill is a stand-alone novel, but I envision it as the first in a series of several books.
That sentence I got from the fabulous Julie Kenner, and it struck me as a sound and reasonable thing to say. However, someone told me to cut it, saying:
Mentioning it could be a series for a first time writer is taboo. I had this tendency beaten out of me very quick.
I'm not sure what the source is for this "taboo". I've never heard of it before. Also it doesn't make sense. Why would an agent not want to know that you have more potential books in you? I mean, yes, the point is selling the singular book that you currently are shopping... but I would think if an agent is going to consider taking you on, they would want to know if you are aiming to be next Stephen King or the next Harper Lee.
Now, in the fantasy and sci-fi genres, the nature of series have changed. Gone, I think, are series like Lord of the Rings or The Belgariad, where multiple books really just tell one big story. At least, from a new author. That, I could see agents and publishers aren't interested in seeing. After all, with one of those you're asking a publisher to commit to three (or five) books that would all have to be published for the first one to make sense. Risky venture.
However, a standalone book that CAN be a series is a different matter. There, you aren't committing your potential agent or editor to doing more. But you are letting them know that more is there, if they want it. That, I'd imagine, is a selling point.