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For production you generally don't need to care about which JavaScript engine is the fastest. Your page should work in all common browsers, period.
However, in certain projects where you are free to choose which browsers to support (such as hobby projects or projects for internal use), you'll find that the performance in different browsers varies a lot.
The two browsers I find to maintain top performance are Mozilla Firefox with its Spider Monkey engine and Google Chrome with its V8 engine. Apple Safari is also one of the fastest browsers with its Nitro engine, new in 4.0. They use new methods for "compiling" the scripts, making them perform much, much faster than before. In the future, more and more browsers will move towards this technology, as JavaScript is becoming one of the main technologies for interactive user content on the web. (I haven't mentioned Opera 10 here because I haven't personally tested it extensively, but I've heard it's faster at running JavaScript too.)
It matters to me as a web developer; many of the tools that I rely on use javascript, and just as a backend developer spends a ton of time waiting for compiles, a front end developer can end up waiting for page refreshes. A fast javascript engine helps speed this up, especially when the page is complex.
Obviously, if your application relies on javascript to be fast for your end users, you must test it on all platforms and make sure you meet rational performance targets- but a fast javascript browser (eg, chrome, opera, firefox 3.5), makes life a much more pleasant experience while in heavy development.
What matters is what's the slowest (that'd be IE) in use by your users. If you can limit your user base to cover only certain browsers, you can increase the utilization of JavaScript accordingly along with it.