Sally mentions this concern in the NT book as being a valid one. This is why she says to always buy organic liver and that even though that may contain some toxic substances, it's nutritive value outweight the dangers of the toxins. I believe that is why she also suggests soaking it in lemon juice for several hours prior to preparing.
Of course, this depends on the quality of the "organic" source, etc. So it is a choice each person has to make/consider individually.
I don't make liver, but I do understand it is very nutrious as in FF's article above.