I modify OBJ's all the time, not to change the geometry but to change the names of groups, or make changes to the material references. I have modified the geometry in OBJ's before, to do scaling and that sort of thing, but I usually have written a program to do it, rather than trying to do it by hand.

I use a text editor called Lemmy, which is a Windows version of the good ol' Unix editor vi. It can handle quite large files, but I don't really recommend it unless you know vi in the first place. It is definitely counter-intuitive for people used to Windows-type editing.