When AREF equals Vcc, no errors can result from varying supply voltage in the R-2R ladder.
Also the choice of the resistors is not critical. It's normally sufficient to pick resistors from the same charge, then also 10% resistors have almost the same parameters. Recently I built a R-2R ladder from 5% 270 Ohm SMD resistors, and it works perfectly in my DAC.
Really critical is the button connection. I'd suggest to place the drivers and ladder near the ADC, to keep the output voltage free of external noise.