In almost every case, entering/forecasting lots of different material expense accounts just makes the process more time consuming and more complicated with no benefit.
If you want to enter material expense by large categories (e.g. by division) to help you forecast your spending according to each division's sales goals, that's fine. But if you're trying to forecast how much you're going to spend on lumber vs. sand vs. perennials vs. mulch - you're causing way more work than you need to. It's very difficult to forecast each individual category since you don't know exactly what kind of jobs you're going to win this year.
Keep your material categories simple, and you'll thank us later. 1-5 categories is ideal. We don't recommend entering/forecasting more than 10 categories.