First the Circle of Death I have found limits me as a sketcher. Scale is important, and yes, I try to keep scale, but with that circle in place I find I am more worried about the circle than about actually sketching and thus the sketching suffers. For a new sketcher learning techniques, I think they need to focus more on technique and mastering them rather than on perfect accuracy. The reality is of the couple of thousands of sketches I've made, most are for my enjoyment and to show me that I have actually improved in my sketching efforts.
I also find that even a thin line impacts the view of the sketch. I simply think the sketch looks better without the border. I did make my mask and will be posting it on my blog in the next several days. In addition I am going to make a couple of practice sketches with it to see if I like it.
Finally, this is my opinion. I think there are two views of sketching. One is where the sketcher really, really wants to be accurate for a study or some other project. Then a circle or a mask can be very helpful. You can see this in this link to Stephen Waldee's article on sketching the horsehead that I did (near the bottom). Here the circle helped me to create an accurate sketch. The other view is where a sketcher wants to capture an object, an observation and wants to objects to close in accuracy but it doesn't have to be perfect or even close. Here a sketcher doesn't have to use a circle and can focus on sketching and on the sketch itself. Often that leads to a greater enjoyment of the process. Sketches for contests, sketches for articles and such need to be as accurate as possible. Sketches to record an observation, an experience, a point at time at the telescope can be accurate, but often it is the experience that matters most. I do both and enjoy doing both and I personally believe if one really tries, without a border circle, one can still be really close in accuracy and it leads to a better view. As I mention, if I sketch without the border, I don't really notice the field stop and I focus on the object I am sketching and keeping it centered and for me, and only for me, I enjoy that experience a LOT more. I hope I answered your question while providing options for others to decide which way they like to sketch. I don't think there is a right way or a wrong way, there is a way that works for the sketcher.
My personal goal if this two and a half month weather pattern of clouds will ever clear up is to go after a few well known objects and then on my blog I'll post a comparison of true field of view and alignment of stars and of the object without a mask or border. I think that will be a very interesting study to do and I have to thank you for the idea.
I haven't had time to go out and do a comparison of true field of view and stop and alignment of stars and of the object with a mask and without a mask. So, the following 3 sketches are sketches I made of the following objects, NGC 1365, a barred spiral in Eridanus, the Rosette Nebula or NGC 2237, NGC 2238, NGC 2239, NGC 224 and NGC 2246, and IC 443. I've sketched two of the three before but this time I felt that the contrast of the Zambuto mirror was so much better that the UHC and OIII filter really stuck these items out.
NGC 1365 Barred Spiral Galaxy in Eridanus:
IC 443 Supernova Remnant in Gemini:
NGC 2237 Rosette Nebula