I think there are several different ways to get there.
But part of fit problems in general is that certain brands and styles don't
"get you" (or me) at any size. I have only been able to get decent fit in a few brands or makers of pants and even then have to have the waist taken up. Sizing down would be too tight in the rear ( and still might not fit in the waist). This is with or without fluid fit.
But just having ease in the seat isn't all there is, either, as some types of "ease" look like baggy bottom, while with others, it is in the right places. For example, I'll see some photos of pants on the forum where the poster has sized up for "fluid fit," and to me they just may not....fit. Both sizes are wrong. Though some of the sizing up might be tailor-able, depending. The rear view on pants is especially difficult this way, and I can certainly relate to either a) abandoning the quest for perfect rear view or b) the skirt and dress advantages.
For being a size up or not, that just so depends on the item and style. If I try on what I think is the right size and it seems too tight, I'll try a size up, but it might or might not fit the way I want it to. If it's too tight in one size and funny fit in the next size, then that top doesn't "get me" . This is typical of even t-shirt type tops where I always feel I'm between sizes, so when I find one that seems to fit "easy" but doesn't look sloppy, then I know it's cut the way I need it.
I'm looking even harder at these types of fit because i don't want my whole wardrobe to swing back and forth --too much time, effort, $$. Plus the fit of one item affects the proportions of all other items worn with it. I need a base of what might be "normal" and flattering fits for me, and then add a few fun different-fitting items from time to time, or keep the basic fit but change something else that makes the type of item look updated.
