.cfg .scn (spacecraft3).ini and even .msh can all be opened, viewed and modified with Notepad safely, in fact use Notepad exclusively for those file types. (don't modify .msh files unless you know what you're doing, just look and learn first)
EDIT:
The reason you don't want to use Word, Wordpad or any more advanced text editor is that it is very easy to accidentally save some font, formatting or other information that would corrupt the file. It works if you're careful, but Notepad already has everything you need.