Most programs either write a full file every time and replace the old file by a single move or append to an old file. Writting in the middle could happen in a program writting to some kind of archive or disk image. There is probably a whole group of programs that do this I'm not familiar with, but I'm pretty sure of my first sentence.
I'm not completely confident (I tried looking in the source and it wasn't immediately obvious) but I think emacs does small in-place edits when you're working with very large files.
