And if you don't want R to add decimals or zeros, you can stop it from drawing the x axis or the y axis or both using ...axt. Then, you can add your own ticks and labels:
plot(x, y, xaxt="n")
plot(x, y, yaxt="n")
axis(1 or 2, at=c(1, 5, 10), labels=c("First", "Second", "Third"))