Let’s look at cross stitch on fabric, this is an example on linen, though any type of evenweave or linen will do.
Is this coverage OK?
I’d say it depends. Here is where I may get a few people mad at me, but let me explain. Many people who cross stitch “allow” the fabric to show through the stitches. By “allow” I mean, it is considered correct technique, and there are many examples of it all over. It is my opinion as a needlework judge that IF EVERY stitch shows the fabric through the stitches, then that shows consistent stitch tension, and I would say it is correct technique, as it is the choice of the stitcher to allow the fabric to show through. However, IF SOME stitches show fabric and others do not, then that shows inconsistent stitch tension and it is incorrect. Now think about how hard it is to make sure that all the stitches have fabric showing through, why it is just as hard to make sure that fabric never shows through.
How does the stitcher resolve this conflict? If the stitcher increases the number of strands of floss in the needle, then the probability of fabric showing through the stitches decreases. Consider this: the number of strands of floss that you stitch with may be changed for some colors of thread and not for others. Darker thread colors may need to have the number of strands of floss increased to provide the same coverage as lighter colored threads. The dyes in darker threads make each strand of the thread a little thinner, so the same number of strands in the needle do not cover as well as a lighter colored thread with the same number of strands. Furthermore, for the same reason, the threads stick together more when using darker colors than lighter, which makes it a little harder to get those strands to spread out for good coverage. Another issue is the contrast in the color of the thread and the ground fabric. If there is a large contrast (dark thread and white fabric) it is much easier to see the fabric than light thread and white fabric.
What I want you to consider is visual distraction. If you are stitching a santa on a light colored fabric and the fabric where the white beard is stitched is completely covered, then the stitch tension and number of strands of thread is correct. However, using the same number of strands of thread in the needle you now stitch santa’s boots with black thread, and the fabric is not completely covered. This stitching is a problem because it is different. Not different because you meant it to be so, but because the tension and thread choice makes it different.
Take a look at this example below, comparing white floss and dark blue floss on a light colored linen. I’d say the fabric coverage under the white floss is better than the blue floss. You can see that the white thread spreads out more than the blue thread. Note the blue arrow - right there is a problem. And just what is that problem? Why, the coverage is complete between those two cross stitches. This will cause a visual distraction because it is different - most of the other blue cross stitches have a little fabric seen between them. Differences pop right out at you and your eye will stop at that place to look at it more closely. When everything is the same, your eye will glide over the area without stopping.
Now this example is magnified. When it is not magnified, there is no fabric noticeable under the white stitches, but it is visible under most of the blue stitches. I would say this is inconsistent stitch tension and fabric coverage. Increase the number of strands of the blue thread until the coverage matches that of the white thread.