What manual of style should I use for technical writing for US Federal Government documents?
Well, before my question gets closed I better answer it.
U.S. Government Printing Office Style Manual is one of the better known style guides.
More recently there has been a lot of talk about plain language, and this sometimes shows up in legislation. The plain language publications tend to have a lot more about English usage.
There are agency specific guidelines, but I'll let someone more ambitious than me track them down.