I think one way I can help to clear question up, and really, to erase it as a "question" at all, is to put depth into a category. Depth is like "amplutude". And that is an aspect of performance.
So, the question, with continued strength training, shouldn't really be 'what depth is good enough' but what depth have I been hitting all along? So, like Joe said, if you have to decrease depth then your bar is too heavy…or something. What does that really mean? It means that depth, as an aspect of performance has not been maintained.
Increasing the load, while decreasing the depth, therefore, does not count as progression.
Your goal is to at least maintain depth during continued progression. At any time, you can work on improving any aspect of performance, so you can work on improving depth. This might mean keeping the load on constant on whatever variation you are working with. Or, you can work on that in terms of "mobility" in general, while still maintaining depth during your loaded training.
Yes I want to question the last line you put. This is similar to what is playing in my mind. If I squat 225 for full ROM and 235 for 3" higher, then is it really fooling around or is it just a black and white issue as you put above? They seem to me two different issues (fooling arond and B&W).
Keep in mind that you cannot hope to always maintain perfect depth (in terms of current performance). You have to constantly monitor performance and make adjustments when you see an aspect of performance suffering, such as depth. During singles, doubles, you should be able to hit constant depth, because you are paying more attention and because the way we do singles has quality closely guarded. With higher volume and reps, not every rep can be expected to be perfect so you look for large perturbations and for the general picture.