OK, judging by this page spread this superstition is true—at least for me. Or at least I shouldn't attempt it if I'm going to get my proportions all messed up.
But that's not the point, and in fact, this superstition is also nonsense. Go ahead, work right across that spread, knock yourself out. Go crazy and turn the book so the gutter runs horizontally as I've done here if that makes you happy—or if you are sketching a really, really tall building or giraffe perhaps!
There is only one time when it is actually difficult (but still totally appropriate if you want it to be) to sketch across the gutter and that's if you're working in a wirebound journal!
But I have friends who work in such journals and they are always working across the page spread, simply ignoring that coil binding.
You can flip through the posts on my blog and find hundreds of examples where I have worked across the gutter to fill the entire page spread with my sketching. (In fact there are probably about 500 and some examples based on the number of posts and the frequency with which I handle my journal this way.) Here's a honey crisp apple bisected by the gutter. Here you'll find a rooster and a building. There must be a hundred examples of Gert sprawled across the page spread like this recent example. And some of you may remember these extreme examples of dogs in a series in my small weirdo journal. The pages in that journal were too narrow for me to not cross the gutter and spread out across the page spread—or so I felt at the time.
You get the idea. You can work right across that page spread and onto the following page spread if you want. (I did this in several places in my Weirdo Journal.) You get to decide.
There are some drawbacks, as my Ibizan Hound shows—when working across the spread. Because of the curve of the pages and the way you end up holding the book (and whether you draw standing or sitting are variables to consider too) you can introduce distortion to your image.
(Let me say right here those ears on the hound are actually too small, it's the nose that needs to be a bit bigger!)
Another drawback is difficulty with certain media. For instance, when I work with Stabilo Tones—which are big fat pencils—it's difficult to lay down color in the gutter as I work across that spread. Happily I typically go in and dissolve those pencil lines with water and move color around with my fingers as well. This means I can get some pigment in the gutter—but you want to AVOID DETAILS FALLING IN THE GUTTER AREA.
You want to do this not only because it is difficult to get the media (even a thin pencil) into the gutter producing the lines you're used to, but ALSO because the gutter is the halfway mark in the rectangle (or square) that is your open book. The laws of composition encourage us to keep the focal point away from the center. (Though those laws can be broken too if you want, but because it is difficult to get media into the gutter I'd suggest in this instance you move your focal point out of the gutter where you can give it the attention it deserves.)
I actually think that working across the gutter over the entire page spread can make you a better person—because it will help you accept distortion, quirky rendering, and other flaws which might float to the surface. You can then set about a course of action to improve. Also it gives you a chance to laugh at your internal critic—"Well of course it's messed up, anyone can see that," you might reply to his nagging.
The best reason to work across the page spread, however, is that it simply gives you more room for your vision, more room to play. That's always a good reason.
Don't worry if working across a page spread is going to make it difficult for you to scan your page spread. Don't worry if your drawings go a little wonky. Shake off fears of getting it right. All you have to concentrate on right now is drawing (or painting). Think of it as another way to exercise your mind-, hand-, eye-coordination. Work across the spread in order to carve out new ways to use negative space, notan, composition, and simply have the expansive sense of fun that comes from filling up the page spread in one go. Exuberant journaling. It's kind of like riding a bike, come to think of it. Go log some miles and go fill up some page spreads.