a lot of people say never trim a leaf from your plant, if it is dead and has done its job it will fall off by itself.
i'm not one of these people, i like to make my plants look pretty and a dirty brown leaf hanging off looks juts that, chop it off!. they can attract all kinds of nasty pests and parasites too and if any do fall off make sure to remove them from the soil, also any leaf that is in contact with the soil will be likely to pick up infections or mold etc.