I prefer quotes for phrases because I think they are slightly more visible and keep the words of the phrase grouped. Also, the mobile site doesn't render italics on my phone's browser, but I don't think that's a consideration for a guideline that applies to everyone. For single words, I think italics are fine but I try to use quotes to be consistent.
You could say "this phrase"
isn't that much different from
You could say this phrase
so I don't feel strongly about it.
I do think however that it would be a good idea to have a style guide thread here on meta that would bring together discussions like Should backticks be used or quotation marks while formatting? and Angle brackets to indicate orthography. It looks like an attempt was made a while ago that didn't get much traction: Memorandum of Formatting