As Rad/Onc is such a tough field to match in, I didn't want to chance not matching and wasting 100s of hours of research, study etc...by being lazy with TY notes. Thus, I mailed a hand-written TY note on nice stationary to each person I interviewed with. Don't know how much it mattered, but I ended up matching my 1st choice. I got the impression from several places I interviewed at that small things like that can sometimes make the difference between otherwise equal candidates.
Good luck on the interview trail.