How to grout in order to minimize uneven tile spacing?
This is really going to be opinion based but I would be looking for a dark blue, tile matching grout. The white grout is going to tie all those shooting rays  of white together and will lose the effect. The imperfections would be minor unless your mother-in-law or father-in-law wanted to make a point in which case no color grout would hide it. Just my humble opinion. You could even think about using the same color for the subway tile depending on your color coordination. Caution: no one has ever let me decorate their house.