Well dotfiles demonstrate that punctuation can have a special meaning in filenames.

I'm not convinced by "quicker to type" arguments as that's rarely the bottleneck, so I'm perfectly happy with using underscores in filenames and variables. I wouldn't use underscore as the beginning character of a filename unless it had a specific meaning to me (e.g. temporary files), so I'd be more inclined to use a two or three character prefix instead.