There is a StartPage Safari Extension available for download and installation from Apple. I would put that in an even higher level of safety than downloading from the developer. Not only has the developer proofed the file, so has Apple.
But of course you are right! I would revise Rule #1 to reflect such.
Just bear in mind that there are apps that offer versions on their developers' websites that differ from those offered through the App Store, and at least some of them lose desirable functionality when d/l'ed from the latter, at least sometimes because of sandboxing.
