@AI: It is my opinion that pencil grade is irrelevant for all of these throwaway exercises. They are not about the marks they leave on the paper but about the experience and knowledge gain happening in the mind outside of verbalization and technical details, about building muscle memory and training the mental model of things, observation skills, creativity improvement, living through the performance, not about leaving graphite trails or any other markings on the paper. For artworks, that's different, there material matters.
Well, tendon and muscle strain is an issue for me, so switching grades is more of a practical 'takes less force' thing. I don't at all disagree about it being about the experience of concentrating on these different things, but I would add that the media you use affects exactly what that experience is. eg. softer grades wear down quicker so you're using a broader tip for more of the drawing -- unless you re-sharpen before you strictly need to. Another example, after I bought this ream of A3 paper, I found I just liked drawing on it much more because it has really good contrast (80gsm + 150 whiteness, compared to the probably 50gsm+ 80 whiteness miscellaneous paper I had been using) so I didn't have to squint at it at all.
I was having serious issues with the Modelling in Ink exercise for similar (tendon + muscle strain) reasons. Partly because I tend to use more force than I need to.. I've been taking the opportunity to train that out, as IME it leads to worse measurement and gesture of individual lines, as well as reduced speed.
(I developed a technique for reviving previously-thought-to-be-dead biros through that, too... "go so fast that you heat up any stuck ink through friction"
Either way, I don't know as for the throwaway exercises I'm using up a bunch of old pencils that just accumulated over the past 25 years or so from all kinds of places. Giveaways from hardware stores, ownerless pencils found on the street, leftovers from pencil boxes. They are all kinds of different grades, Bs, HBs, 3Bs, 2Bs, some are of really grainy/crappy/broken mine quality and not even marked. Also not using the same one each day, just grabbing any random one, usually one that I don't need to sharpen, so that I can just start drawing.
If there's a fluctuation in appearance, it's likely due to fluctuating mood, coffee level, random pencils, varying scan settings, different pressure and or speed while drawing or anything else but again, that's all irrelevant for the throw-away exercises.
It's pretty consistent actually, after that date everything has more contrast. But yeah, that particular change could easily be attributed to switching HB -> 2b or unbreaking scanner settings.
Apropos of 2b, I bought like 100 2b pencils from Woolworths for $10 (2 packs of 50). I am no longer worried about running out of pencils for gesture work.
It's funny how you say 'coffee levels' like it's some kind of RPG attribute.
3h a day was killing my back/neck. 
Hmm, is the centre of your display pretty much aligned with the bridge of your nose?
I found this reduced my neck strain from 'frequent' to 'basically never' (but to be fair, my posture has also improved over time)
.. alright, I do have a few pictures I can put here.
