Aware of http://radio.linux.org.au/ and http://parnass.com/ , I'm
still wondering, as a general rule, what brands of scanners are more
likely to have Linux software available?
From desktop scanners to handhelds that include some transmission
bands, to plain old car two way radios that aren't even scanners.

Any simple rule, like "don't buy a winmodem" we can apply here?

A short of brands should be easy to remember just in case the
opportunity arises and one must make a quick decision.