my posts frequently include things i learned while teaching and topics of discussion with other teachers who are the rock stars in calligraphy. the teachers i talked to about slant all agreed that most students have a natural slant. an older student with many years of writing at a particular slant, will do better -in the beginning- if they adjust the exemplar to the slant that is already embedded in their hand-eye-brain.
with italics, if you write with no slant at all, and have been writing with no slant for 40 years, then, it is a good idea to try a no-slant italic -in the beginning. after you learn all the basic strokes and can do a good job with no slant, then you can start adding in the amount of slant that pleases you. with pointed pens, if i had a student whose regular penmanship had no slant, i would start them on upright ornamental and they would usually do better with upright -in the beginning- and after some success with upright, they could move on to other styles with slant.
so, in a nutshell, if you already have a specific slant to your writing - use that slant to learn any style and AFTER you achieve some success with a basic hand done at your natural slant, move on to adjusting the slant.
then there are the beginners who have regular penmanship that does not conform to any slant. it can be challenging to get them to slow down enough to maintain proper slant. if they are motivated, they do fine and they have the flexibility of choosing any exemplar and learning any slant, because they do not have any pre-determined natural slant.
IMHO, there isn't one way to proceed. the student has to understand and embrace the tedious process of learning many different tasks - and then do them all at the same time:
the tools - papers, nibs, inks, grip (each of those can be full of details)
ergonomics - if you are not at the right height, you'll have trouble
the basic strokes - essential
slant - essential
how the strokes connect - essential
white space - uber essential, but frequently the least understood component for the beginner
so there are at least 9 things that you have to work on - at the same time - that IMHO are essential to making progress.
neglecting any one of these can be the equivalent to driving your car with a flat tire - yes, it can be done....but, it's not advised.
a final note to the folks who love the wild and crazy, no-rules, no regrets, just noodle around and have fun path - yes, that's a valid approach. if you are going to go that route, be honest about your intent and realistic about what you think the results will be.
guidelines or no-guidelines is a bit like the recipe or no-recipe debate. it would be rude to tell the classically trained french chefs that their approach is a waste of time and home cooking tastes better. and a classically trained french chef has respect for the rustic cooking that comes from people who never read a recipe in their life, but they turn out delicious food.