Tamping should be a constant - don't change it. If you want to pull different length shots, adjust your grind and dose, but not your tamp. Tamping is hard enough to get 100% consistent as it is, don't fuck things up by using different tamp methods.
I understand the argument for the "classic" espresso. I'm even happy to concede that anything outside the parameters of 7g of ground coffee poured to 30ml in 30 seconds shouldn't be called espresso... So long as you are equally happy to concede that anything that strays from the guidelines set forth by the "Associazione Vera Pizza Napoletana" should not be called pizza!
The way I see things is that coffee being an organic product (as opposed to manufactured), you can't expect it to follow a strict set of rules. The fact is, some coffees will present better at a lighter/darker roast, extracted at a lower/higher temperature, ground finer/coarser, dosed higher/lower, and pulled longer/shorter... And thats the beauty of it! Finding the right balance of all these variables to extract the best from a particular bean is an adventure and an art.
So... Traditionally there were really three "sizes" of shots - the Ristretto, the Normale, and the Lungo each at 15ml, 30ml, 60ml respectively. With this in mind, if you are following the guidelines of a "classic" or a "real" espresso then yes, there are long shots and short shots. However even speaking in classic terms you still wouldn't change your tamping pressure or technique to make these properly - you would adjust the grind to either increase the dose and restrict the flow to make a Ristretto, or you would open up the grind to allow a faster longer pour for the lungo.
Now back to "organic" method! We've already established that my methodology will produce a range of sizes for a standard espresso based on which bean you happen to be using - Maybe the coffee is coming out at its best at about 22ml one day and another day presenting best at 33ml. You can still adjust things to pull an outstanding Ristretto shot or Lungo shot. Again you wouldn't be adjusting your tamp, but there are countless other things you could be adjusting!
Okay, now I don't have time to write more! So I'll leave with this: Depending on who is talking to you, there is no right answer... or there is - 7g/30sec/30ml and everything else can't be called espresso! It's up to you, really. People will tell you all kinds of different things. Asking an online community of coffee folk anything will result in countless different answers and expert opinions. Everyone works in different shops, in different countries, with different cultures, and different practices. It's really up to you to find your own answer. Yes, there are short shots and long shots... and no, there is no such thing as short shots and long shots!
You know what my advice is? You can forget everything else I've said, making the last 30 minutes of my life useless(dammit!), but take this on board - It's all about taste. If you want to find your true answer try absolutely-fucking-everything until it tastes right for you. When you make a long shot, does it taste good? No? Then that's your answer.
Find an approach that you feel represents what the espresso is, and what you want to present. Pull this shot, and base everything on this shot. You'll probably find that you need to tweak this a little from day to day to achieve a great shot, but probably not dramatically.
If you do for some reason want to offer both a normale and a ristretto, see if you can accomplish this just by increasing your dose of ground coffee for the smaller shot. You may have to make a grind adjustment as well, in which case it would require a second grinder to be practical.
Of the 3 easily-manipulated prep variables available to the barista, dose and grind have so much more impact on the shot. Develop a solid and consistent tamp technique and don't change it.
Try developing the relationship between how much your dose weighs and how much your liquid shot weighs. Work with different doses, grind settings and aim for different yields. Form your opinions around those things.
Alex, thanks for saving the three of us a lot of time! (honestly, I was prepared to post a very long response, so I sincerely appreciate it)
The first rule of espresso is that "There is no standard." Not officially, anyway. I have my own opinions. Your opinions may be different. My "ideals" do not always agree with the coffee in the grinder with which I have to work, so adjustments are often necessary.
Every coffee is different. Every machine is different. Every grinder is different. Every barista is different.
Before defining how to achieve a result, we must first figure out what exactly it is that we are trying to accomplish. Hopefully, we are trying to serve a very (VERY) tasty shot of espresso. Now, how to achieve this goal will depend entirely on the coffee, the equipment, and the barista.
As much as my ideals want to agree with "it should be extracted to Xoz from Xg of coffee in X time", a good barista will always adjust to what's in the cup, and adapt to improve on it in some way until the best flavor is achieved as can be possible from the variables at his/her disposal.
That having been said, preconceived notions about what an espresso "should" taste like can often get in the way. Try to keep an open mind, and let the coffee speak for itself.