In one of my former lives as a structural steel detailer (guys that
draft the shop drawings) we always used a consistent spacing for the
pickets. The odd spaces were at the posts, equal at each end of each
section. This way you never had more than 1/2 a space at each end.

There are building codes that dictate the maximum space between any
railing. In the old days it was stated as something like "Nowhere shall
a 6" ball be able to pass through any space in the railing".
Interestingly, the 6" was dictated by the average size of a babies
head. No kidding.

This commercial approach is obviously the most productive but I
believe it is also the best aesthetically. In my career I was probably
witness to at least one hundred instances of this.

BW