How careful should I be about sending my spit out to one of these companies (Ancestry, 23andMe, etc.)? I don't know how seriously to take the warnings about the risks of having my genetic information in some company's database. (Hi, I don't know shit about science or information security.)

Ancestry now says on their site that you can ask for your results and your sample to be destroyed, if you want...

Trustworthy?

--
the untold want by life and land ne'er granted
now voyager sail thou forth to seek and find