Italic manuscript and "standard" manuscript are just 2 different fonts. You would choose to give instruction only in one (unless you have a very interested/artistic child and choose to do the other more as "art"/calligraphy, almost.

).
I chose my manuscript (and cursive) instruction options based on what I was comfortable reproducing myself; what I was familiar with.
Do you print/write in a script closer to what's offered in A Reason for Handwriting and Cheerful Cursive (see Bigger for cursive options), or is your handwriting closer to the Italics print and cursive?
The reason I chose what is closest/easiest for me is that there are times when my kids need a model - "Mom, how do you write ___?" If I have to go check a book somewhere or page we've posted on the wall, I find that cumbersome and I'm likely either to be annoyed by the question

or to give them a model of what I remember (which may or may not be accurate for what they're learning/have learned in their book). If the font they learn is close to my own handwriting then it is much quicker to give them a model, plus, they'll find it easier to read my writing in the long run.
I personally don't see a reason to teach both, nor to switch from one style to the other. I suggest you plan your manuscript choice now in view of your cursive choice for Bigger, as well.
It's totally a personal choice which you pick; no right or wrong choice.

(In fact, I believe there are regional variations as to which is more common in certain places compared to others - I never heard of Italics until I started homeschooling.)
Blessings,