An easy, loop-free alternative is to use the horizontalalignment Text property as a keyword argument to xticks[1]. In the below, at the commented line, I've forced the xticks alignment to be "right".
n=5
x = np.arange(n)
y = np.sin(np.linspace(-3,3,n))
xlabels = ['Long ticklabel %i' % i for i in range(n)]
fig, ax = plt.subplots()
ax.plot(x,y, 'o-')
plt.xticks(
[0,1,2,3,4],
["this label extends way past the figure's left boundary",
"bad motorfinger", "green", "in the age of octopus diplomacy", "x"],
rotation=45,
horizontalalignment="right") # here
plt.show()
(yticks already aligns the right edge with the tick by default, but for xticks the default appears to be "center".)
[1] You find that described in the xticks documentation if you search for the phrase "Text properties".