Published authors say it shouldn't be done, which means, I guess, that it's possibly, just not advised.
By 'isn't finished yet' do you mean it isn't finished being written, or it's still part-way through it's third or fourth heavy edit? Because if you mean isn't finished being written, then I would think it still has a lot of work yet to be done on it, as most novels need quite a bit of editing before most agents/publishers will look at it.
If it's almost finished being edited however... well, then maybe starting to query isn't as bad.
Since publishing, I've spoken with several other agents and publishers other than those with whom I signed. I got mostly the same feedback - that I was lucky in signing my first novel. They all said that it is not only preferable, but usually their requirement to see a finished product (in terms of top to bottom, not completely edited) when submitting to a publisher. They don't want you to send it all at once, but rather a query with a few chapters; however, they want the rest of the work ready to see if they consider you publishable and sellable.
I went backwards. I hard many non-fiction and creative non-fiction works floating around. I signed with a publisher to help an athelete write his memoir. While talking, they asked my favoirte inquiry to date...."Do you have anything else you're working on?"
I sent them half of a novel and chapter sketches for the rest and they signed it. That was a year and a half ago and it got published not long ago.