My reasoning is that "horoscope" is mostly there to anchor the idea to something people "get" already.

Yep.

Also, I'm liking the ease with which one can say "my horoscope told me to...", either to oneself or to others who're aware of the project. 'Bites' doesn't work too well with that, and neither does 'fortune cookies'. I guess it's minor enough that those could win out on the grounds that they don't make one sound like a nutter.

(We're using 'Actually Useful Horoscopes' for the dev phase, but it's easy enough to change tumblr account names.)