The decision about when to do a transfusion is, to some extent, individualized depending on the situation, what symptoms the person is having (if any), the cause of the low hemoglobin, the rapidity of the drop in hemoglobin, and how low the hemoglobin is. Doctors also differ about how low a hemoglobin can be before the person needs a transfusion.
Therefore, an exact number may not tell the entire story.
Here are a couple of older threads dealing with anemia, low hemoglobin, anemia treatment, and transfusion:
http://www.medhelp.org/posts/Hepatitis-C/Anemia-and-dose-reduction/show/1693075
http://www.medhelp.org/posts/Hepatitis-C/Hemoglobin-levels-and-anemia-on-Incivek-Inf-Riba/show/1694740
http://www.medhelp.org/posts/Hepatitis-C/Whats-the-lowest-Hemoglobin-level-before-transfusion/show/373812
Here is a link to an article about transfusions in certain situations, but it is not specific to Hep C treatment. The type of anemia people get with Hep C treatment is hemolytic anemia. The article may or may not be of help to you.
http://www.wadsworth.org/labcert/blood_tissue/redblood.htm
I cannot find any specific article or guideline for when to give transfusions for anemia during Hep C treatment. If I find one, I will post it.