I'm just wondering if there's a way(either by using regedit or any "hack") to make explorer windows(and perhaps apps windows) to automatically fit vertically by default instead of using win+shift+up keys
maybe Eusing's Auto Window Manager is for you. while AWM doesn't do exactly what you have in mind, it will add a whole rake of new options to the default windows management, such as program start rules, alignments, transparency etc.
AWM is freeware and works with Windows 7.
Well, you can simply use the top resize edge of the window and drag it all the way to the top of the screen. This will autofill the window vertically. See AeroSnap.