When printing, 300 PPI is the best print resolution for digital printers. The design of a file with a resolution below 300 PPI will result in blurring and pixelization. On the other hand, you should not design with a resolution higher than 300 PPI. As far as the eye can see, there will be no difference in print quality. It will also make your file much larger (increase file upload/transfer time).
We always recommend 300 PPI drawings to print, but we also know that sometimes you do not have the ability to edit your drawings. If you cannot print your files with a resolution of 300 PPI, you will have to take some measures to ensure that a lower resolution works for you.
We recommend that you set the resolution in your graphics editor (i.e. Photoshop) to 300 PPI before you start drawing your file. Increasing the resolution after the start will still result in blurring/pixelation.