I've done a little research and it seems that there is a "right" answer to this question.
First of all I agree that this can be premature optimisation, so profiling before deciding to switch is essential.
The graph above was generated in excel, using a normal distribution, and testing the space overused by various initial list capacities, using 10,000 samples and a mean of 10,000. As you can see it has several interesting features.
- For low standard deviations, picking a bad initial capacity can waste up to eight times the space of the best choice.
- For high standard deviations relative to the mean, less savings are possible.
- Troughs, corresponding to the lowest memory wastage, occur at points dependant on the standard deviation.
- It is better to choose a value from the right half of the graph to avoid list reallocations.
- I couldn't find an exact formula for the minimum wastage, but mean + 1.75 x standard deviation seems to be the best choice based on this analysis.
Caveat: YMMV with other distributions, means etc.