Actually, keep 3.
The first is your equipment... the stuff that you use over and over. Why? So that when you sell a bride on a 6/9/12" wedding cake, you actually HAVE a 6", a 9" and a 12" pan (or 2)! And when you go to your cake supply store to get a few things, you don't buy your 18th #2 tip when you REALLY needed a #1.
The second inventory is your consumables. Your cake ingredients, your colours & dusts... your boards & boxes, dowels and drums. Your business cards. Your contracts. Your office supplies. Tape. Ribbons. Pre-made gumpaste (or other) flowers and decorations. This one is the list that will save you money. Just do it! You will discover that you can stop buying 13" drums, but could really use 14" ones. You will discover that you really do go through a LOT of boxes, and it might be worth buying the big case after all.
There is nothing less professional or more frustrating that not having something you should have, or spending money on something you already have. If you know what you have on hand, you can "sell" someone on a cake that uses some of those decor bits that you have had sitting around forever, or a piece of equipment that you haven't used in a while.
Then there is the insurance aspect. You DO have insurance for your cake stuff... RIGHT???? If anything were to happen, a detailed inventory will make sure you are adequately covered. You might be surprised just how much cake crap you have.
An inventory is also an excellent way to help you prune back when you run out of space! Knowing that you have 3 different (but similar) petal veiners means you can chose to get rid of one or two. Even remembering that you have that <
If you're anything like me and desperately need to get organized, an inventory will also help you plan your space! Knowing that you have 43,000 dust pots tells you that you will have to allow a bigger spot for them. Knowing that you don't have a 9" round pan means you don't need to allow a spot for it at all. Unless of course you NEED a 9" round pan. Then you can allow a spot for it when you get to order one!
The third inventory is something very different. I touched on it ages ago in a post. Keep an idea inventory! Notebook, folder, envelope full of scraps of paper... doesn't matter! Get an idea for a cake... sketch it! Write notes! WHATEVER!! But once you forget (and you WILL forget)... it's gone. Then, when you need an idea... you have nothing.
Keep an inventory, and you're set! If you are really organized, you could have full sketches made for cakes you want to do. Next time you have a bride in, start by showing her those sketches! She might just order it!!!