A common question when matting photos or artwork is whether to leave a space around the image and if so how much.
Whether to leave a space or not is dependent on a couple of things. First of all is the work signed or numbered in the white margin below the image. If so - a space at least as wide as the written signature and number should be left all around the image.
Secondly - is the piece very dark and are you using a dark mat. If so - the white space may create a glaring white border that visually takes away from the image - in which case you will probably prefer to mat directly up to the image.
You should also decide if you are using a double or triple mat and if so -the number of layers of color or space may make the design to "busy" if you also leave white space around the image.
When designing your mat the uppermost concept should be that you wish to attract the viewer's eye to the artwork not the mat or the border surrounding the work.
Often that alone determines your decision.