I still follow that rule, perhaps somewhat anachronistically. This has been long optimized in CPython so it's not quadratic:

$ python -V

Python 2.5.2

$ python -m timeit -s 'v=["a"]' 'v.append("a")'

1000000 loops, best of 3: 0.199 usec per loop

$ python -m timeit -s 's="a"' 's+="a"'

10000000 loops, best of 3: 0.127 usec per loop

The string append is actually FASTER now. Tried with strings of length 5 and it is still faster.