Is 'doubling' really crucial?

that's the golden standard.  It means there is enough yeast activity in the bulk (first) rise to continue on with the recipe.  If the dough hasn't progressed this far, it is good to keep in mind that the times stated in the recipe are too fast and that you must wait for the dough to show you it is ready for the next steps.

While I must agree that it is the "gold standard", IMO it's far too difficult to judge when looking at a mass of dough.  I usually recommend stopping just short of "doubled in volume" or, as a preferred alternative, using the finger poke test.

First make certain your covered dough ball is in a warm and entirely draft free location.  Even if you can't sense a draft, it may be present so be aware of the environment you select for proofing.

The finger poke test should not be used for freshly shaped loaves. 
That's because the resulting response from the dough will give you a false positive result.  Waiting 20 - 30 minutes before doing the finger poke test will give you more accurate results.

If the hole remains entirely, without gradually springing back to the surface, you've proofed too long.

If it responds completely and restores the surface of the loaf to its original condition, it hasn't proofed long enough.

It should rebound about half way between the depth you poked and its original surface condition.

Sometimes there are variations: a few recipes specify triple, and sourdough is quite often more like 150%.

But what's easy to communicate, easy to measure, and typically about right is "double".

As to why 200% rather than something like 225%, let me ask and answer a different question instead: How important is the "optimum" bulk rise amount? I think the answer is "not very, double is 'good enough'".

When working with whole grain doughs I always let the second rise go beyond double. 

Gabe,

There isn't any way I know of to determine what percentage the dough has expanded. Oh I suppose you could use a tape and take measurements but to many variables. We are left with common optical sense I'm afraid. Once a long time ago I tried to do some poke tests every 10 minutes so I could understand what the perfect amount of proofing looked like. My test was inconclusive because by the time it looked about double, the poke test was giving me the same results. I gave up on the poke test. Some swear by it but I get mixed signals.

So to answer your question, no I don't think it's crucial that it double. I mean, if I'm making Ciabatta and I want a light airy crumb I know it has to have a good increase. When I make Italian bread I bake 2 loaves on a sheet pan and as they grow, they get closer to each other and the sides. At some point I know they will join which isn't a good thing. In that case, 200% might be to much. This notion of proofing is complicated. Now that I have a home proofer and I can maintain a stable even temperature that I have adjusted, it seems to be easier. Back in the days of boiling water in the microwave oven or turning the light on in the oven, everything was an approximate average. The ambient temperature was rising or lowering constantly and so part of the dough was always behind the ambient. It's complicated and a very good question. Of all the aspects of baking, this is the one that requires the greatest amount of judgement based on experience. You'll get the hang of it if you are observant.

Eric

Judging the amount of first/bulk rise need not be difficult. It's true that if you let the mass of dough expand in all three dimensions, you can have an awful time with tape measures and calculations and the like, and still not get a good answer. And it's true that judging when the second/proof rise is done is more difficult, and can impact the amount of oven spring and the height and overall texture of the finished loaf. But there's an easy, straightforward way to judge the first/bulk rise with "good enough" accuracy. (Watch out to not mis-apply suggestions about the second/proof rise to the first/bulk rise:-)

For the bulk rise, put the dough in a straight-sided container you can see through that's sized so the dough completely covers the bottom. (Some use plastic containers, some use glass. The container need not be absolutely transparent, just something you can see light and shadow through easily. And approximately straight-sided is usually the best you can do.) Put a rubber band around the outside of the container at the level of the top of the dough. Then let it rise. When there's as much dough above the rubber band as below, that's double.

This of course only works on the first/bulk rise, not for the second/proof rise of "shaped" loaves. For that, you'll need the finger poke test and/or some judgment. Note you need to wait half an hour before the first finger poke test; if used immediately right after shaping, the finger poke test quite often gives a false result.

If you do some stretch and folds in between won't this effect how long it takes to double ?