I'm weird and overly cautious with these things... so keep that in mind. =)
I personally don't bring ANYTHING in from outside. It's a risk (however minimal) that I'm not willing to take. I know plenty of people do, and have fun luck... however I stay clear of that.
As far as toxicity... I just don't trust it. It's bad news to use it with a lot of invertebrates. I'm not fully educated in the finer points of WHY exactly it is toxic to some animals. (ie- hermit crabs)
Ten billion people can tell me it's perfectly fine and safe, but if there's other options that are even slightly better- I tend to go with them. I think of it like this- A piece of chicken from KFC versus a piece of chicken from open range, organic, well cared for chickens you cook at home.