Ha, good point, and an interesting question: What kinds of dashes did Dickinson intend?
It's a hard one to answer: We could look at published Emily Dickinson books from the time, but did Dickinson really pay that close attention to or have that much control over the type?
We could look at Dickinson's actual personal documents, but if they were handewritten, distinguishing dashes could be difficult even if there was intention there.
Fortunately we have troves of her handwritten documents; all of her poems were first printed posthumously. To me, she's using the punctuation as pacing or tonal markers as opposed to ligatures ("I'll clutch— and clutch— " vs "I'll clutch-and clutch-"). Many publishers style these marks as longer than normal m-dashes for that reason, which makes sense seeing as they are rarely used as asides.
Em-dashes have been the norm in every Dickinson poem I read, and I think it might have derived from the preferences of Victorian publishers, who I understand loved those long dashes.
I imagine it would have been up to the typesetter to make the call. The conventions for dash usage are fairly straightforward. You use em-dashes for asides, en dashes for ranges, and hyphens for most other cases. Its easy to figure out the right character from context (apart from en ranges vs hyphen ranges).
You want Robert Bringhurst, poet and typographic nerd. He gives them special withering attention in his Elements of Typographic Style. I think he referred to them as Victorian excrescences?
