You can view your drawing in Sketch in two ways. Which way you prefer will depend on the kind of work you do. These modes can be toggled in the View › Canvas menu by clicking on Show Pixel Grid. Note that when you’re at 100% zoom (actual size) there’s no difference between either mode, but you’ll see it when you zoom in.
If you care how every individual pixel in your document looks then this mode is for you. What you see is basically the equivalent of first exporting it to PNG and then zooming in Preview.app.
If you don’t care about this then the default view is best for you; when you zoom in you’ll see everything still with smooth curves.
Forced Pixel Preview
Sometimes though you’ll find that you cannot disable pixel preview even though you may want to.
This is because the Blur effect has to work on raw pixel data. This means that the only way Sketch can display these effects is by rasterizing your vectors an then applying these filters. This does mean however that a vector-preview is no longer available, so we’re forced to show pixel preview.
When you try to disable pixel zoom and it can’t, it’ll offer to show you which layers are preventing pixel zoom. On large documents this can be quite a timesaver.