I think pre-washing the fistula with antibacterial soap then swabbing the buttonholes with alcohol pads and/or antiseptic and gloving up is KEY.
My husband has been self-cannulating using the buttonhole method for a little over 2 years now. He was hospitalized last summer for a staph infection but (per a vascular surgeon, his buttonholes were not infected). The disease control doctor told him it was from a common bacteria found on everyone's skin and to not beat himself up over it. By the way, ever since then he has gloved up before he sticks himself...
The most recent study I could find online is an old one posted
October 26, 2012 from the CKJ Clinical Kidney Journal titled "Arterio-venous fistula buttonhole cannulation technique: a retrospective analysis of infectious complications" April 24, 2012 And no, his doctor or Neph has not mentioned any recent studies about the risk of buttonholes and endocarditis and they haven't made him sign a form stating as such.