Right now, the optimal way to shape fuel tanks is to have one big fuel tank block, since two or more fuel tanks make the craft heavier. This is very sad, as you're sacrificing prettiness for gameplay advantage. I would like it so that if the fuel tanks are connected to each other, they would be more or less considered as 1 block.
Or another alternative, having a "Bottom" "Middle" and "Top" version like a fairing, so that you can shape and color it as you want without adding more weight. The Bottom section could count only the bottom cap weight, the middle one doesn't count the bottom and top weight, and the top one doesn't count the bottom weight.