It isn't so much the word count that is important. It is a measure of progress towards a goal. I have novels of 70,000 words and 125,000 words, and each is a fully fleshed out story. Some just need more than others. That aside, I have spoken to some Sci-Fi and Fantasy publishers, and they prefer novels of 100,000 words that have scope to become series. Readers are drawn to novels that use the same universe. The first book of the Dune trilogy is about the same length as the next two combined. Further books are all of relatively equal length, as are the Prequels co-written by Frank Herbert's son. I don't know their word counts, but I would guess that the first book is over 100K, while the post trilogy books are around 100K.
The novel that I'm working on now is the third in a series. The first is 100,000, the second 70,000, and this one is 106,000 and still going, although I may have finally found my ending, which will probably take another 10,000 words or more. A further draft is likely to reduce those numbers, although I often find myself adding description, which I often neglect in the first draft. On the other hand, I also generally use too much dialogue.
I'm not counting individual words, just progress towards that "ideal" of 100K, but I usually know around 30K how long it will be. My last novel, which I wrote as part of NaNoWriMo, looked for a long time like it would be about 65K, but finished up around 85K. I don't add or remove words according to word count.
