Unless you are striving for an eclectic antique shop look, groupings of pictures should look as though they belong together.
Select frame styles that are compatible and matting styles that will work well together and create a balanced, unified look to the group as a whole.
Spacing of your pictures is also an important element in picture grouping. Each picture should be placed not too far from nor too close to each other.
The most commonly recommended way to create balance in a picture grouping is to lay the entire layout on the floor. This will allow you to get a sense of how the pictures will look together on the wall, allowing for adjustments and accurate measurements. Also you should align the top or bottom of the various pictures in the group.
This accurate placement is critical and very easily achieved with a good Picture Hanging System.