[Janice D. Soderling]

To keep it simple, each person should decide which will be the style guide of their choice and adhere to the rules therein.

That rule also has an exception as the writer may encounter a publisher who has other "style guides of choice". If you know who your publisher is ask them if they prefer MLA or CMS or ODWE. Each has house rules that they adhere to.

Chicago Manual of Style has 21 main entries under the index word "italics". Among them:

for dynamics in music writing, 7.75
for emphasizing words, 7.49, 7.57
for foreign words, 7.51-55, 10.93, 10.131
for word's first occurrence, 7.57

any or all of which might be pertinent in this case.