I'm retired as a IRS employee but I worked in another office on a different application. When (you use IG, it's really called TIGTA, they get to carry guns) calls you in, you have the right to have your union rep there, but I have never heard of or known of your direct manager there.
As far as a low level IRS employee, his or her manager is a low level manager, a (glorified clerk) and that managers direct superior is a (glorified superior clerk). Every low level person at the district office level has to follow to the letter, rules set forth in the Internal Revenue Manuel (IRM). Only Washington D.C. has the ability to change the written procedures that all IRS employee's MUST follow, for their evaluation, for extra vacation days or bonuses. If a low level employee didn't follow IRM rules and that person's low level manager let it slide, then that managers low level superior let it slide, it would be IMPOSSIBLE, for anyone from that level down to make a policy that varied from the IRM. That could only come from Washington or a actual change in the IRM.