This is also the point where you have the highest leverage. Your leverage is always based around the idea that you might go to another company. When you're considering multiple offers, it's very credible that you might work for another company. Once you've been hired and you've been working there for a year, they figure the inertia of leaving your job and going to another company is big, and limit your salary raise.
You need to be careful with your advice to others, I know you mean well but it could really mess somebody up in a very real way.
I actually use a small tweak: rather than "all are equally good so I'm choosing based on salary," I pick the company I want, then go to the others and say "another company has offered me X. Could you offer me a little more?" Once they do, I go back to the company I want and say "another company has offered me Y. I really want to work for you, so could you match Y?" Perhaps the "I really want to you," and only asking to match another offer, is more important than I thought?
However, I think using post-offer leverage to negotiate extra pay is a bad idea. The leverage exists, but it's limited. I'd rather use it to get insight into (and favor in) project allocation than a measly few grand. Project allocation is the difference between having a successful career-- learning a lot, working with great people, getting the promotions-- and needing to find a new job in 12 months. A few thousand dollars is nothing in comparison. This style of negotiation is more subtle, and takes the form of a "reverse interview" than a line-in-the-sand "I want $10k more" statement, but I think the former is better at (a) presenting yourself well, and (b) getting you what you actually want, which is a successful career.
I also tend to subscribe to the philosophy that the first 120 days of any job (including post-offer negotiation) are an extended interview. What you want to show is that you care about your work intensely and that you have the potential to reach very high levels, but also that you expect to be challenged and given appropriate work; not that you are a mercenary who makes career decisions based on a few thousand dollars.
Salary matters-- you need enough that money isn't a personal problem for you, and salary is an indication of how the firm perceives your professional maturity-- but small margins don't.
False symmetry. For a company, an extra 5k per year shouldn't even make a blip on the radar. If it does then I would have to question the stability of said company. For me however, as my sole income it does make a very obvious difference.
I suppose, though, I've never tried to get into Facebook or Google. And I suppose in those companies, with huge applicant pools, they'd chose somebody else if I indicated I only saw working for them to be equal to working for my other offers (unless, of course, one was one of them, and my other offer was the other).
