What's the idea behind flagging .t files for taint checking?

I write most of my code with tainting enabled, so I want to make sure everything works with tainting enabled. It makes my life easier if other people do it too :-)

Any good rules of thumb for when it's the right thing to do?

My default is to always have tainting on.

I wouldn't have thought it a necessity for a test script, unless the test was specifically testing the taint-safe behavior of something.

And how do you know which bits of your code (and any of its dependencies) relate to taint safe behaviour? :-)

You can either spend a lot of time and effort figuring it out yourself, or you can just tack '-T' to your perl line and get perl to figure it out for you.