Paying attention to your pixels when your creating your icon, even though you're probably working with vector graphics, is extremely important. No matter how you create the artwork, it is going to be displayed to the end user in bitmap format, and thus it will be pixel-based. If you're not taking care to make your designs pixel-perfect you'll end up with blurry edges, which leads to a very unprofessional product.
To give you an example of how this can create a quick sample using an icon I created a while back. On the left is the finished icon that I shipped. On the right is a version with lines I didn't place very well. As you can see, the lines that are meant to represent writing look very blurry on the sloppy version, and it looks pretty awful.
This happens because the edges of those lines don't line up with the pixel grid. In Sketch when you zoom in to about 800% the software displays a grid over your work. This grid corresponds to the pixels in the export. Sketch also has a setting under "View" called "Show Pixels" (also, this can be activated with 'control + P') which will give you a live preview of what your icon will look like drawn as a bitmap image.
It's a simple fix really. You simply need to zoom in and check every edge. The easiest way to do this is is to check each shape's size and position values (detailed here) and ensure that they're whole numbers rather than decimals. This will give your icons a professional level of polish and professionalism.