There are cases where it's dangerous, such as:
```
>>> def foo(x=[]):
...     x.append(1)
...     print x
... 
>>> foo()
[1]
>>> foo()
[1, 1]
>>> foo()
[1, 1, 1]
>>> 
```
In the instance above I suppose it shouldn't matter, but I've tried to start using `list()` just to be safe.