For the more traditional sword arts, there are very definite set angles that are used in kata or combat, but for the cutter there are no such restrictions. Indeed it is advantageous to be able to pull whatever angle cut you want, out of the bag at any time as and when you need it. The targets we cut can be in all manner of positions, of various mediums and/or in motion and this provides a lot of variety and complexity. Although we train the standard 4 diagonals, 2 horizontals and on occasion 1 vertical cut, there are also all the different angles in between that we may find ourselves requiring in order to make any one of these cuts happen.
Accuracy with these sudden cuts needs to be as pinpoint as we can get it in training, because we can almost guarantee it wont be when we're actually trying to make that cut happen. Also, when we decide to make a cut at a certain angle, we need it to happen almost without thinking, because if we deliberate for too long, the chance to make it happen passes and we miss the target. When we suddenly decide that we're going to reverse the direction of the cut to catch a bottle that fell the wrong way for instance, it's a split second decision. We need that accuracy ingrained as much as possible to make up for the fact that we're dealing with this unexpected occurrence.
Although dry cutting all these angles can definitely help you with this and I'm a big advocate of dry practice, I have found that some targets are better than others for various reasons. You want a target that isn't too hard to cut, but one that requires speed. You also want to be able to make multiple cuts on this target before having to replace it and you want this target to be cheap/easily sourced.
So once again, enter the humble 500ml soft drink bottle. It doesn't matter if its coke,pepsi or whatever, they're an excellent choice for this espcially as you can just use your already cut bottles. If you want to fill them with water to give them some weight to hold them down then that's fine and I find a bucket of water near the stand helps with this. If you don't want to then thats fine too, getting multiple cuts on empties without picking them back up off the floor is harder but thats another challenge isn't it?
The idea is to try to take as many little chips out of the bottle as you can until it's too small to cut further. Downward cuts on these targets are easiest and upward cuts are decidely more difficult. Try it and you'll see. Horizontals.. well I haven't managed one yet.
I personally found that this practice of reusing empties and trying to slice the tiniest bits off the top of the bottles, helped with my irritating habit of always managing to cut about 2cm above what I was aiming for. This is just another idea that you can use to recycle your already cut bottles and it trains tip speed/acceleration as well as edge alignment and those all important angles.