How high do you overspec your resistors?
the power spec is not really a power alone specification per se, you gotta think of it as heat as well, if the resistor has proper cooling it should be fine. However, depending on the application you spec everything with a certain safety margin, I think for high grade circuits that would not be acceptable since they are in the range of 50% or so

Now, you gotta consider this can you afford the circuit going down and needing that resistor replaced? can your other components handle a short at that point given that resistor burns out? do you have enough space for 500mW instead? what is the role of that resistor to begin with... it takes a lot of considerations.

you are right at 10% safety margin, which for most common uses would be okay in my opinion, for something like military or circuits for automobiles it would not be acceptable design though. 

you gotta look at each situation and assess the cost, risk, reliability and decide the margin with all of that in mind. Blindly using a margin is dangerous as well.