In my experience, salary negotiations are rarely done face-to-face. Usually they are handled by telephone and/or email. Naturally, you can have whatever you want in such circumstances.
That said, if you are going into a face-to-face (or video) meeting to negotiate salary, I generally agree with the recommendations that you don't want to take a calculator into the meeting. Know the minimum you're willing to accept before the meeting so that calculations are not necessary.
Before the meeting, think through how you will value your complete compensation package, as only you can decide how valuable a benefit will be. For example, to me, an extra week of vacation per year would be worth somewhere between 5% and 10% of salary (assuming I'm getting a market value offer), rather than the roughly 2% you'd expect in terms of time worked. Some people will find it valuable to have schooling paid for while others would have no interest. Having a good idea of this sort of thing ahead of time is better than figuring it out "on the fly". Also, it will make you look more prepared, which could even affect the negotiations; at least you'll avoid negatively affecting them by looking disorganized.
That said, let me offer a possible alternative if you feel you must be ready to perform some calculations: A tablet (e.g. Google Nexus or Apple Ipad). If you already use one for note taking, that would allow you to make your notes and you could do some quick, simple calculations. However, if you're going to do that, make sure they're quick and unobtrusive to the process - if you have to say something like "Excuse me for a minute while I run some numbers", it's probably too long.
Also, some portfolios, such as this one (not an endorsement, I just grabbed a random one from a search) come with basic four function calculators built in. This also would allow you to have something for quick, simple calculations without being obtrusive.