Your rates are one of your most important business components. After all, they are a big part of whether you will get the job or want the job. They determine whether you need to work more or you can enjoy some more free time because you charged enough or more than you need to meet your income goals.
First, it is important to understand that the true rate, no matter how it is phrased, is hourly. At the end of the day, it takes you time, not words or sentences or paragraphs to complete a task. The reason many translators charge "per piece", is that it is probably the most accurate measurement of what is to be translated. If you really needed to, you could count each word with your fingers, and there would be no misinterpretation of what a word is. You would, however, have to specify if the wordcount would entail source or target words, especially in a language like Spanish where there is documented shrinkage when Spanish is the source text.
Since I mainly address the agency market, as it is what I'm most familiar with, there is not a lot of leverage when negotiating. You have your rate; they have their rate. In my experience, you could expect to gain one penny at most. I know, there is no guesswork for me when posting my rates. Based on the agency and where it is based, I am pretty certain how much I could charge, and sometimes they just come out and say: "We pay such and such per word."
When bidding on a job, I have three different rates: high, medium and low. I apply the high rate when I don't really need the job. I apply the medium rate when I am interested, but could afford to not be awarded the job. My low rate, and mind you, it is within decent parameters, is when I really need the work and I know that it will be easy and fast to translate.
Ideally, the client should have to ask you your rates, so that you aren't tied down to a specific figure. However, when bidding on jobs, it would be impracticable to keep the client guessing, and they would most likely just move on and negotiate with the next translator. Therefore, I make it a practice to include my rates in my cover email, so there is no delay or confusion in the bidding process. This is always worked for me, and I recommend it.
I have been asked if rates should be listed on a translator's resume (CV). I invariably answer no, because I find it to be a document that tells clients about one's work experience, and not specific business details like rates. At any rate (no pun intended), I'm not certain that clients actually read my resume, but I do know that they read might introductory emails.