I've seen meditators of many different schools talking about the importance of not walking away from a "bad" sitting. All books I've read reflect this commitment. All monks I've heard too. Conversely, I never heard or read a single meditator to take this issue lightly as in "well, if you are not much up to it, sure, maybe some other day".
There are certainly a number of reasons for abandoning a specific practice. A big one, is that we might be doing it in some improper way, or in a way that increases unwholesome states, hindrances and/or decreases faculties associated to meditation.
"As, Anuruddha, I was abiding diligent, ardent, and resolute, I considered thus: 'Excessive meditation upon forms arose in me, and because of excessive meditation upon forms my concentration fell away; when my concentration fell away, the light and the vision of forms disappeared. I shall so act that neither doubt, nor inattention, ..., nor perception of diversity, nor excessive meditation upon forms will arise in me again."
-- MN 128 (Bodhi Trans.)
On a personal note, this is how I worked with this issue: if I identify the unwillingness to meditate as a hindrance, I do not make concession. Otherwise, I stop, or do not meditate -- at least formally, sitting. (I'll make an adendum to this at the end of the post).
For example, in an individual sitting, if it's just hard, I'm in discomfort, or I wish to be doing something else, I make all the effort necessary to continue until my clock tells me to get up.
If I'm in "unbearable" pain, I stop -- I had to discover what is "unbearable" pain for myself, so today I'm roughly aware of how much I can push making progress (and without turning the meditation into a nightmare; been there, done that, a lot!).
Similarly, if I start a meditation carelessly and then feel very hungry, I prefer to stop, have my lunch and then do a proper sitting -- no need for unnecessary suffering. But only as long as this is the real reason, and not a hindrance. I remember a few times when I had to interrupt a sitting, but two reasons compelled me to do so simultaneously. One was very reasonable, the other was a hindrance. At one time, I only interrupted once the hindrance subsided. Another time, I wasn't able to do so, and I got up. And I could tell I got up because of the hindrance, not because of the counterpart. I blamed my inattentive mind and this compelled me to be more careful and practice more.
I've observed that this -- not giving to hindrances -- is extremely important for two reasons:
Giving away to hindrances may hurt future sittings considerably.
Personally, I know all I need is one single "walk away" followed by some half-baked excuse in my mind to make the future sittings much harder -- as it will be much easier to stop.
The difficult sittings (the ones in discomfort, with a strained, confused, non-concentrated and noisy mind, in "non-ideal" places or circumstances, etc), may be instrumental for upcoming progress.
I've personally had experiences with periods (days, months) of meditation that seemed like walking in the desert, going nowhere, and individual sittings that felt just terrible, but were so very important.
Years later, I noticed the "desert" was due to me not knowing some basics of meditation (for instance, what to do with hindrances...as a zen trainee, unfortunately, I was never introduced to these basic concepts and what to do about them). In other words, I'd say I was unskillful.
But when skills are in place, that's the whole point of meditating when hindrances are present, however strong: we're supposed to master them. And, I guess, that is what makes these difficult sittings feel so special; when we sit again later and it's....something else entirely.
So, I think once one develops, for example, some minimum skills with hindrances and is able to roughly replicate a "deeper state", these terrible sittings are seen just like the first week at the gym: a warm up (or simply actual work: even though we are displeased, muscles grow stronger -- I've observed the same happening with mind). Provided one is exercising correctly, otherwise it is just painful and possibly damaging.
So, revising (and concluding) from what I stated earlier: I would continue the sittings if (a) I'm seduced to not meditate because of a hindrance; and (b) this is the n-th sitting (for some small n, like 4) that is not good / without progress; otherwise it probably means I'm doing something wrong, and I better figure that out, perhaps, by other means, like studying.
