Those all sound like good reasons, and, once upon a time, I would just buy a pattern, make it, and amazingly, it would fit. I don't remember doing a lot of measuring or fitting, but I was young and thin, and yeah, it just worked.
Now, I try on lots of RTW dresses, and they don't fit. Why? Because I'm not young and thin anymore, and my body is different sizes all over. I can buy separates, but when I want a dress, it's pretty much guaranteed that I'll need some alterations.
So, back to sewing. I've been studying the Fit for Real People book, and it's all about the tissue fitting. I've done the first fitting of my 1960s Twiggy dress. I'll spare you the picture of me in my underwear wearing half of a tissue dress, but it was enlightening, and yep, needed some fixes. Mostly to the yoke which was too long, and the shoulders were too wide.
I've made a 1/4" tuck in the front and back yoke (for 1/2" less), and I redrew the outside of the shoulder/armhole seam. I haven't tried on the new version yet, so I'll have to get back to you on that. And I know I'll have to make some adjustments to the sleeve. Later. Later. I still might lower the neckline in the front because it was a little choking, but I want to see if the tuck helps with that. Oh, and I added extra tissue to the sides so that below the waist I can have a 1" "just in case" seam allowance. The tissue appeared to fit, but I don't want it too snug.
It's all about the fit.