And, all of this said, there really aren't any 'rules'. A publisher will publish a good book, whether it has been put out there or not. Ultimately, the best thing you can do for yourself is write the very best book you can.
Also, don't be afraid to try. You won't burn any bridges, so long as you're honest about your book already being in publication. Dean Wesley Smith has some words on that:
http://www.deanwesleysmith.com/?p=4131
There's a lot in this post that I find contrary to my experiences, and some misguided from my perspective (though he and I are obviously coming from different perspectives), but I'm not going to dissect it or trash it. It's another perspective, and the OP can read it and compare Mr. Smith's advice and career path to the career path of other authors and decide which path to follow.
But, I will agree with Mr. Smith that self-publishing likely will not "kill" a career - hardly. But...it may very well make the specific self-published book unlikely to be picked up by a trade publisher if it doesn't sell well as a self-published book.
OP, do your research.
Burning first publication rights is not the problem it once was. In fact at BEA this year a lot of people were discussing that self-publsihing is starting to become the new "proving ground" for authors. It actually helped Michael's deal but he sold very well in ebooks first.
Not sure who's opinion this one was. It's not what I'm hearing from most in the trade end of things - it is what I'm hearing hypothesized by some promoting the e-publishing and self-publishing trends, and I don't think anyone can point to more than the exceptional cases - and that's my point. Sure, if your self-publish or e-publish and sell exceptionally well, sure that might help. But the vast majority of people who self and e-publish are not going to have those exceptional sales.
Read what
Amanda Hocking has to say about banking on the exceptional.
So, sure, it's not a career killing move, but self-publishing or e-publishing a specific book can't help that book get published by a trade publisher unless it sells exceptionally well.
If it sells 100 copies or less, as many, many, many self and e-published books do, it will actually make it harder to get an agent or trade publisher to look at it at all.
So...why make it any more difficult? There is nothing that e-publishing it or self-publishing it can do to add value to the book unless it sells very, very well. Exceptionally well.
If it's of the type and quality of book to be published by a trade publisher, then self-publishing or e-publishing it won't make it any better or more attractive, and likely will make it more difficult. So...
Again, if you want to be published by a trade publisher, then exhaust those efforts before you try self or e-publishing that book. But, if you're willing to give it a go at e-publishing or self-publishing, without any concern for trade publishing, than do your homework and make a strategy for doing so.
I'm not saying don't e-publish or self-publish - hardly - I'm just saying if the OP's ultimate goal is to have this particular book published by a trade publisher, he/she should try that avenue first, before e-publishing or self-publishing this specific book.
~suki