I'm using the dual extension .ctx.tex as a way of telling my editor (emacs) to invoke the context mode of AUCTeX when I load a file with that extension.
Auctex automatically recognizes plain tex vs latex vs context files by the first few lines in the file.
The main time when that method does not work is when starting a new file. Since almost every tex file I make these days is a ConTeXt file, I set the default for that case by using this .emacs line:
(setq TeX-default-mode 'context-mode)
`Not all who wander are lost' (J.R.R. Tolkien)