In general, I've found that if an epub looks OK in ADE, it will display quite decently on my Sony readers (350, 950, T1). It will also usually display passably well in these Android apps: Mantano, Aldiko, Bluefire, Sony, Nook, Overdrive. The Kobo app tends to add margin to that which is already in the epub, so it's not quite as good on display as the others. Mind you, all I've tested are fairly basic, mostly text epubs.
The exception was when I put an epub with a couple large images on the Sony, i.e. 1500px wide high quality JPEG and a 1500px wide PNG (small in file size) and they came up as blank pages. Display was fine in ADE. Reducing to 1200px wide solved the issue. So the Sony readers may have some limitations on image size.
If you get into the apps that offer a lot of customization options like FBReader and CoolReader and Moon+, those may not display your book quite like you styled it. The goal for me personally is to just be sure my epubs are readable in those apps.
I also test Kindle .mobi and .azw3 conversions. There are times when valid, working CSS does not translate in a conversion. I had KindleGen barf on some simple shorthand CSS once.
It would sure be nice if every epub reader app or device could be consistent in display, but that's not real world right now. I think I even found one reader, a cloud reader called Fabrik that didn't display images. So test, test, test as best you can and realize there are going to be compromises, because you can't control what your reader will use to read with.