The way it should work to make it fair across the board and force teams to learn more maps is to have a random 3 to 4 maps each month for each size of match. For instance....
1st month for 2v2 or 3v3 have small to medium maps like pbcup, back2back, disaster, and shaz22. For a 4v4 or 5v5 have larger maps such as pbcup sandassualt, renoir, shaz33 and the likes. Make it straight random. If you leave it to random chance then neither team can really complain. It all boils down to how hard each team practices. With switching maps once a month or every two weeks the teams that are willing to go the extra step to work together and learn them will have the advantage. If you allow teams to input which maps they want then there will be complaining and excuses. Make it completely random. Too easy.