Does black iron pipe need to be cleaned before installation?

My answer is yes threads need to be clean. However it probably depends on what is used for sealing. Black pipe is normally used for gas. I don’t want any leaks and pipe dope both gas rated and regular state the threads need to be clean and dry. I don’t remember ever seeing info on white or yellow tape but I have wiped the threads as I use oil when threading so it has become a practice to wipe fittings for gas if I thread them or not and with either tape or dope.

Mill varnish is applied at the mill ,each mill may have it own type. Threads are cut after mill varnish is applied . If there is oil on the threads it is likely from cutting threads so cutting oil which should wipe off with rag or solvent. Mill varnish is usually more difficult to remove.