Normally, you will 3D-print your keyguard with its back surface against the build plate of your printer.  That will reduce or eliminate the need for support material and ensure the best adhesion to the build plate.  However, there will be times when it will be a better choice to print your keyguard upside down, with the face of the keyguard lying against the build plate.

One example is where you have added a sloped edge to your keyguard.  The images below show the same keyguard in two different orientations with supports turned on in the slicer software (supports are in green):

face-up	face-down

You can also achieve a much smoother top surface by printing the keyguard face-down on the print sheet – particularly a smooth sheet:

However…

This approach only works if the top surface of your keyguard is flat:

• the screen area of the keyguard matches the overall thickness of the keyguard
• the rails between the cell openings are wide enough – with chamfered edges – to provide sufficient surface area to adhere well to the build plate
• no ridges
• no clip-on strap pedestals
• no raised tabs

If you can meet all of these requirements, you should experiment with printing your keyguard on its face to limit the amount of support material needed and to produce a smoother top surface than could be achieved with a typical 3D-printed top surface.