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.