Good clean copy is your first step toward getting your manuscript accepted.
Like most publishers today, we use the electronic file the author sends us to set the type. Whether you send a CD to a mass market publisher in New York, or e-mail a small publisher a copy as an e-mail attachment, your publisher will use your own data file to set up your book. That is why it is super important for authors to turn in error-free copy and to follow submission guidelines, or, failing that, at least to be consistent in how they prepare the manuscript.
Don't "type" your copy as if the computer were a typewriter, putting a return at the end of every line. Yes, some people still do that, but it leaves a lot of unneeded returns in the file and if they're there, your publisher will have to go in and take them all out by hand.
If you live outside the USA and are applying to a US publisher, it's simple courtesy to set your spell check to US English for that final check.
The biggest favor you can do yourself, or your potential publisher, is to be consistent when you type the manuscript. If you use the tab for paragraph indents, use it all the time. Don't tab half and use the space bar for the other half. That way if the typesetting program inserts it's own indents and the tabs become double indents, we can search for the tabs and replace them with nothing and automatically remove the problem.
There was a time when editors read and made notes on your copy, when typesetters took that copy and typed it in, when proof readers read for errors and copy editors checked grammar and facts, but today -- even in the big houses -- less and less of that is going on. More and more publishers depend on you, the writer, to send in copy that is ready to print.