Check a boolean before setting it in a loop, or just set it

I have a rather tight loop with the following check to see if balance had ever been positive:

balance_null = True

while (crazy_loop()):
    ...
    if 0.0 < balance:
       balance_null = False

In no place would balance_null ever be set back to True. Would the code be more efficient if I were to check the state of balance_null before setting it?

balance_null = True

while (crazy_loop()):
    ...
    if 0.0 < balance and `balance_null`==True:
       balance_null = False

The code example is in Python, but I am actually interested in the general case, which is why I'm not simply benchmarking it myself. What is considered a best practice, and what are the exceptions to the rule?