Adjustments need to be made after interpolating a weight from 2 masters. For a Grotesque typeface you can just slant the whole typeface at once (if you have it in Illustrator that is) turning it into an Oblique. Design a one-story 'a' and 'g' and you pretty much have an italic already. I personally still like to adjust letters though but it won't be anything dramatic. I mean, if you design a serif italic then the process is a lot more complicated then adding a slant and replacing a few glyphs. With a humanist sans serif you might also play a bit with different slants for different characters just like in the oldstyle serif italics but for a Grotesque I like to keep it quite simple.
As for condensing by 2%, this is not a practice I really like to advice because in principle it's bad. If you auto condense a character horizontally, the vertical strokes will become thinner while the horizontal strokes stay thick, thus messing up the letters. This is especially lethal to serif fonts because they have even more horizontal strokes. The reason why I auto condense my sans serif italics by 2% is because I still like my italic to be ever so slightly thinner and more narrow. I condense it by only 2% so the effect I just explained won't show. I could manually condense and thin the characters but that takes quite a lot of time. If the effect were to show up in characters with horizontal or vertical strokes I would of course adjust the letters. As long as the end result looks beautiful and consistent I don't mind breaking rules in type design. I think it's the rule-breaking which allows progress and change.
Hoezo heb je eigenlijk een lettertype gemaakt? Was het een school opdracht?