Good point, Harry. Thank you. I should have thought of that.
As it turns out, both escape codes work, but display incorrectly in the Book View within Sigil. Both will work and display correctly in the Firefox ePub plug-in. (I use Bookman Old Style for the font.)
The Alt+2011 approach does not work, however. That's OK with me; I actually prefer to use the escape codes anyway.
Since it appears to be simply a font issue, is there a way to change the font in Sigil? That's been a minor issue for me, but this bumps the priority a bit.