Because most Photoshop designs or specifically the layer comps within those designs tend not to fill the entire canvas you normally end up manually cropping whatever format you export. So I extended on of the layer comp export scripts to do a crop as well. I'm export to PDF but I'm sure you could add the lines below to crop the image exports too.
var cropBounds = duppedDocument.artLayers.getByName('ExportCropSize').bounds; duppedDocument.crop(cropBounds);
The first line get the outer bounds or a layer called 'ExportCropSize' and the second line does the crop. A nice extension to this would be to get the reference to the layer that denotes the crop size from the name of the layer comp. So a layer comp called 'LayerCompOne_CROP-DefaultCropSize-CROP' would use a layer called 'DefaultCropSize' for the crop size. It's too much for my needs at the moment as I only need to crop to a single size per PSD.