Statistical accuracy of multiple resistors in series or parallel
This is a great idea, but if you know anything about statistics when calculating the mean you need to know what your statistical distribution is. Resistor values do not always fit into a normally or uniform distribution. This will make a difference when your calculating the average because you could have a bias.

In layman's terms if you have an 1% error and you sample 10000 parts of a batch of 100 Ohm resistors, you might measure the value of the average to be 100 or you might measure it to be 100.3. (where going to assume that the distribution we picked is a good representation of the entire sample size for simplicity sake) The reason why is because manufacturers don't always guarantee the bias or the distribution. It would be a problem for you because if your desire is to average two values to get a better value, a bias would not help you get closer to the average value. You may be able to find a manufacturer that will have a histogram of resistor values (I think I've seen them in the past), but you are at the mercy of the manufacturer. You could also check the histogram yourself and some people have done that.