In the ones I have done, I always kept the distance between the balusters no more than 4" apart.
The reason is to prevent a baby from sticking his/her head through and getting stuck. It's usually a code requirement.
Just cut a spacer block out of wood at about 4", then do a dry run without fastening the balusters.
If you like where the last one ends up (Not too close to the next post, then go with it.
If you don't like where the last baluster ends up, then cut your spacer block down a tad bit and do another dry run.
Most upright posts don't have an exact equal distance between them at the top and bottom
As you are fastening each one, keep measuring the distance between the top of the baluster and the post you are working towards, and then measure the bottom as well. If there is a slight discrepancy, then fudge just a hair with each top or bottom to get them to come out right.
