Like Mitch, I pull to a target brew ratio. But I also agree with George that you can't just do that.
In my opinion, and that of others, watching for the "blonding point" is not a reliable methodology for determining when to cut off the shot. The so-called blonding color is a difficult thing to describe, and can vary depending on the coffee and preparation technique/equipment. Further, opinions vary on where the exact blonding point occurs. Opinions also vary on the effects of cutting off the shot before or after that point. Many people have reported that shots taste better when they are allowed to run past the blonding point, and I'm sure there are others who have had exactly the opposite experience when they've cut off a shot early or when they think the shot has gone blond.
Sure, you know when the flow is very dark that the shot hasn't run long enough, and there's an obvious point at which the flow turns very light and you know that you're past optimum extraction and are entering the realm of over extraction. But we're not talking about obvious extraction flaws. We're talking about where to cut off the shot when it has reached the range of acceptable extraction, say in the window of about 18%-22% extraction yield.
Mitch and I, and a lot of others on this board, have found that it's more reliable to pull to a particular brew ratio. The target beverage weight and the time it takes to get there are easy to repeat. The technique is to pull that brew ratio and, as George says, taste the result (and, as Mitch and I might do, test it with a VST refractometer.) If the brew tastes like you think it should, or better than it did with any other dose/grind setting, then you're done. If it tastes or measures under extracted or over extracted, you need to change the flow rate by adjusting grind and dose, or the shot time, or combinations thereof, to get back in the window. But with each iteration, you're shooting for the same brew ratio.
Once you find a flow rate that produces a balanced shot, you can fine tune the grind/dose to produce the optimum flavor while maintaining that flow rate. Even if you change grind and dose, when the flow rate is held constant, all you have to do is watch the scale and the time to know when the shot is done. It should be balanced because it was when you previously pulled at the same flow rate. You don't have to look for a color change.
Now, one thing to bear in mind is that the brew ratio mainly determines the strength of the espresso. After all is said and done, you might have a balanced shot with excellent flavor, but it's just too weak (Lungo) or too in-your-face (Ristretto). If that's the case you can change the brew ratio and dial in again.
Finally, the technique I use would not be appropriate for serving a line of customers in a high-volume cafe. It could be used by the head barista to dial in a particular coffee, and then the line baristas could either run the shot for the specified time or cut it at the color indicated by the head barista. I suppose when scales are eventually built into commercial espresso machines that might change things.
The bottom line is that pulling to a brew ratio is a technique that some (many?) of us find to be a more reliable way to consistently reproduce a given shot than trying to do that by watching for the blonding point. It is by no means a substitute for using taste as your guide for setting all of the shot parameters.
Hope this makes sense.