After making a shot (or multiple shots) of espresso, there appears to be a lot of activity going on. The best I can compare it to, visually, is a pint of Guinness settling.
Is it best to allow the shot to complete this 'settling,' or is this unnecessary as nothing really happens in this ~30 seconds after pulling?
Not necessary in my opinion. In fact, i prefer to add it as soon as possible. The activity you describe is the crema dissipating. I prefer a lot of crema... if you wait 30 seconds, there should still be decent crema integrity. By the way, Starbucks trains their baristas to add the shot within 10 seconds of brewing.
The answer is dependent on the coffee you're serving. I've worked with several blends from different companies and have found that some are better with resting than others. What's key here is the flavor added from the aromatics, or the crema. I've worked with very light roasted espresso blends and found that if I let it sit for about 30 seconds, the flavor was more pronounced. With a more medium roasted blend, I found that if the coffee wasn't consumed quickly, the pleasant flavors dissipated.
