I think it's important to remember when we're deciding where to stop the shot that it doesn't start off as good espresso, and then turn in to blond swill at a specific point, there's a number of different stages, and oftentimes none of them will be great by themselves. When I'm dialing in a tricky blend I'll try to take a taste from the beginning, middle and end of a shot, and I usually get something like, in order:

1. Cool, strong (usually fruity and/or pungent) flavors, oily mouth feel
2. Creamy, tasty, but simple flavors, sometimes some acidity or citrus
3. Lighter and watery, sometimes the end is nice and floral, sometimes it tastes like old dishwater, definitely hotter

And I'm sure that if I was quick and/or organized enough I could find at least another stage or two in between, especially towards the end where the flavors seem to change pretty quickly. When deciding where's the best place to cut the shot I try to line up visual cues with changes in the taste. Sometimes I like to let the shot go a little blond (or cut the shot after the flow has visually become less voluminous and more watery if the color doesn't change much) if it will round out the flavors, and sometimes I'll cut the shot at the first sign of the flow changing in anyway.

Of course this means that sometimes I'll end up with a tiny little puddle of espresso in the bottom of my cup, and sometimes my demitasse can barely hold all the crema, so there are other factors to consider, and trying to figure out how temp, grind and dose affect all the different stages is quite beyond me. But sampling the shot as it's pouring when I'm dialing in some beans has definitely helped me learn how to spot a good time to cut the shot when I'm pulling a couple espressos the next morning.

Blonding is easy. It's a colour/consistency/crema-stain combination that is really easy to identify, once you've gotten used to it.

It's also good to remember that these Golden Rules are not be-all, end-all, 'This Is How Espresso Is Made' inviolable laws. These are the basics that will get a newbie off the ground with his new kit. Once he's airborne, he can learn bush-landings, barrel rolls, loops, and power dives all he wants.
The Golden Rules of Espresso are a starting point. If you follow them, you will get pretty decent espresso every single time. Once you know the rules, feel free to break them. To suggest that they are not the best course of action for every machine for every coffee for every taste as a reason to discard them is just a bit silly.
Use these rules til you feel that you've outgrown their constraints, and then spread your wings and fly.

A couple of years ago Andy S was making and selling "Schectermatics" as a benefit for the Coffee Kids charity. These were plastic bars milled out to hold a line of 6 shot glasses (and he included the glasses). You pulled the bar while the shot ran, so that you had distinct samples of espresso from each stage of the shot.

The biggest lesson to me was that the last sample wasn't foul tasting at all. It was simply thin and dilute, which means that letting the shot run past the blonding point mostly dilutes it. This may or may not be a bad thing, depending on your personal preferences.

Story with photos http://www.aldocoffee.com/2007/06/playing-with-th.html.

Randy G. wrote:The point being, there is no rule. Many of us have been saying for a long time that the so-called "rule" is merely a starting point to be used by those starting out, or possibly tuning a new grinder or new machine. From that starting point the fine-tuning of the variables comes into play with only one thing that matters - taste. If all you do is stick by quantitative science and ignore the art, then all you have is a cup of hot brown liquid.

This.

30-in-30 is a reference point, and yes, it is based on 30ml including crema and 30 sec from pump start. It does not apply to exceptionally fresh and gassy coffee, or old and dead coffee; nor does it hold for chopped/naked/bottomless portafilters. Is it the "ideal" extraction? Nope, but it sure is a good place to start looking for one.

Related discussion on James Hoffmans blog: Dialing in without timings