The chances of a political problem for an American joining a tour would be very, very small. The most dangerous thing is the chance of accidents. The trade embargo affects the safety of planes, and road accidents are way too common. This is a risk of any travel, however, and I would be aware, but not so afraid I would stay home.
There is a chance that events could change, so staying aware of the political situation is important. I would recommend a book by Terrance Ward, Searching for Hassan for some insight on what it might be like.
I just thought of another excellent book from the perspective of an American family that returned to Iran for a visit after living there during the McCarthy blacklist era.
It is called Searching for Hassan, a journey to the heart of Iran, by Terrance Ward.
I didn't say it was a comic book, but it is.
Many of the books on iran are popular not because they're accurate but because they correspond to and verify a pre-existing preferred narrative, and Not Without My Daughter was the maximalist version.
So Persepolis is pushing her own agenda and making false claims ie: "virgins" have to be raped prior to execution in iran because "Islam prevents them from going to heaven" -- which is total rubbish and atrocity propaganda.
Similarly, http://www.washingtonpost.com/wp-dyn/articles/A60490-2004Jul18.html
read more: http://weekly.ahram.org.eg/Archive/2006/797/special.htm
Other books that didnt' get as much attention https://www.amazon.com/Searching-Hassan-Journey-Heart-Iran/dp/1400032237