I find it really helps to keep them evenly BECAUSE of inventory space. They can share gear, but also the materials will be shared. Most materials have a given range of level they're used in. For example, when my crafts were near level 50 I predominantly used cobalt, electrum, rosewood, and raptorskin. I didn't really need to carry other materials besides ones used in that range. Now, if I had some at 50, and some at 30, I'd need to carry both level 50 materials AND level 30 materials.