Imam Hasan al-Askari
Hasan ibn Ali ibn Muhammad (c. 846 – 874) was the 11th Imam of Twelver Shia Islam, after his father Ali al-Hadi. He was also called Abu Muhammad and Ibn al-Ridha. Because Samarra, the city where he lived, was a garrison town, he is generally known as al-'Askari ('Askar is the word for military in Arabic).
I am not personally aware of narrations specifically encouraging the visiting of those sites. (Someone may correct me!) However, there are ziyarat texts narrated for them.
However, there are narrations encouraging the visiting of all of the Imams, which would include them, and it is good to go and pay respects as a gesture of loyalty and to help maintain a tie with the history and people who once walked there and were laid to rest there. Some people might also want to go as a form of resistance to the oppression that Shi'a face today and to make a stand to show that we will not be afraid to go, and that is also good.
That being said, there are a lot of narrations emphasizing visiting other places, such as Mecca, Medina, Karbala, Najaf, and Mashhad, so I would personally place the emphasis on visiting those places first.