Although the phrase "sign off" is not generally seen as a euphemism, it could be seen that way. In the context of Watergate, the phrase "sign off" means "to approve of." We might speak, for example, of whether President Nixon "signed off" on the burglary and bugging of the Democratic headquarters and/or on the attempts to cover up that incident.
In such a case, you could say that "sign off" is a euphemism because it downplays the extent of presidential involvment. The phrase "Nixon signed off" may not carry the same emotional weight with a reader or viewer as the phrase "Nixon approved." It makes it sound less as if Nixon was actively involved in the process.
So, this is a euphemism to the extent that it makes it sound less as if someone like the President was actively involved and more like things just sort of happened.