Do to the fact that Inventor doesn't handle pen styles, autodesk is in the process of helping us with an ilogic rule that will match materials to layers. From what I can gather, layers will control the color and lineweight based on the material. For further information read the post "Assign Layers to Materials" this should help get you started.
From my past experience, I am no longer utilizing idw's. I use the Inventor DWG file as my standard now, this will make it easier for anyone to open the files from within mechanical desktop or basic autocad. In application options, drawing tab, set "Default Drawing File Type" to Inventor Drawing (*.dwg) This is just my opinion and you may test it out and see if it would be a good fit for yourself or company.
