Random thoughts -
My personal rules are that (a) there are no rules, and (b) context is everything. Specific substitutions may or may not work, depending on how strongly meter is established by preceding lines, and the voice of the poem - brisk and conversational, or formal - it can all make a difference.
Personally, I am far more comfortable varying the rhythm - caesuras, breaks, enjambments, dashes - to add life to a poem than in deliberately sliding in substitutions. If a substitution occurs accidentally, and it sounds reasonable (context is key), I'll go with it.
I will sometimes deliberately inject a substitution - even a clinker - to underline something in the poem, to offer an awkward reading to accentuate an awkward moment. Context (did I say that?)
I also find that I tend to be more metrically faithful in rhymed verse, particularly fixed forms, and less so when I am writing blank verse. It just seems to feel right that way - nothing deliberate.
I do use headless lines fairly frequently - sometimes for effect, to start a line a bit more brutally; and sometimes because it follows a feminine line and I almost always follow a feminine ending with a troachaic (or headless, or whatever you want to call it) start, because - well, because my ear thinks that's the natural order of things.
Final point. I get a kick out of these discussions on substitutions because, almost invariably, if I use a substitution - particularly in a fixed form - one or more critters are going to note the metrical anomaly when I workshop
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