People often ask me why the edges of their laser cuts are not square. The laser beam is being focused at an angle to a spot, so no cut can be perfectly square, but there are things that can make it worse. Note: All of the images are exaggerated to show the affects.
You first need to understand how the lens works. Laser cutters use a collimating lens. This means it takes parallel rays from the beam and focuses them to a single spot. For a couple of complicated physics reasons, it can never do this perfectly, but it should do it close enough not to be a factor in this discussion. Below is a picture of a collimating lens. A typical beam width is usually about 5mm-8mm and a typical lens is about 20mm-25mm wide.
You can see that the beam forms an hour glass shape. This can cause a little angle. With a 6mm wide lens and a 50mm focal length, this angle is typically 3-4 degrees.
To get the least affect on your part, you might want to center the focus in the middle of your material.
If you are getting a bigger angle than a few degrees, it is more likely because the beam is not in the center of the lens. The lens will still focus to that same point, but the hour glass is at quite an angle to your work piece.
This type of angle is offset in one direction, so you may see it more in certain directions of travel. If the beam is moving from right to left in the above image, you might not notice the problem at all.
Does a longer focal length help? It can, but due to the complicated physics issues I referred to earlier, a longer focal length creates a larger spot size, which reduces power density. See this calculator.