When mounting artwork I always like to weight the mat bottom about 2 cm more than the top and sides which I typically make10 cm (4 inches) each. I do this on almost everything I frame unless it would look bad due to the shape of the image. Sometimes even more weight on the bottom on small items. It is purely a visual thing and has something to do with the way our eyes work together. However, one client did say it's because during Victorian times pictures tipped out from the wall and that's why some pieces are weighted.
A landscape piece, especially a small landscape does not tend to look as good weighted. A portrait piece on the other hand usually does look good weighted. If the art is bold, the mat needs to be larger to contain it and it needs more weighting. If the art is long and thin, it does look good with the sides one dimension, the top a little bigger and the bottom even bigger still. A square composition can be weighted or not. Each will look good. Also if the arts subject is extremely heavy toward the bottom of the composition, bottom weighting on the mat looks better.
There are of course NO RULES when it comes to mounting. You can try the oriental approach for instance with equal top and bottom mat margins that are far larger than the sides, or extra wide side margins with equal but narrower top and bottom ones.
It's a matter of doing it and seeing how it looks. Having said that though, most of the time, weighting looks better than something not weighted.
