"But would you put a spondee in if it clearly put an extra stress into the line? Would you put a pyrrhic in if it clearly made the line lack a stress?"
No, I don't think you'd do that. But I also don't think it's all that easy to find a case where it "clearly" operates the way you describe, since I think a reader attuned to meter will generally "feel" where the beat belongs or doesn't belong, and register the proper demotion/promotion accordingly, if the basic metrical pattern is otherwise established. I think Susan hits the nail on the head when she refers to conversational tone -- it is there in the conversational tone that the identity of this or that beat might be obscured or ignored, since conversational tone is what it spoken in reality, whereas the metrical beat is something that underlies the conversational tone and often corresponds with it but sometimes is just an inertial sort of feeling that serves as an underpinning or counterpoint for the "felt" meter.
I wouldn't automatically disapprove of a line like "aCROSS /the LINE /of STRAIGHT /TREES in / the SNOW" -- there's a tension, to be sure, with the "in" wanting to take a beat, and feeling as if it is taking one (at least to the part of us that is tracking the metrical underpinnings), while "trees" battles to take the emphasis (and probably does so for the part of us that is tracking the conversational tone), and the effect can, in theory and in context, be just the sort of effect that is called for. This kind of thing has to be handled with great skill, of course, and often sounds bad because the skill is lacking, so it's safer to avoid it (as I do), but it's the kind of thing I believe people like Tim Murphy manage to pull off fairly often.
Interesting, of course, but you obviously have a practical handle on all of this in writing your own verse, so it's probably best not to overthink it.
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