Hello olly
Windslammes and other similar devices will make a difference, but whether its good or bad depends on the outfit and how they are set up.
Not all outfits will benefit, and it is not easy to give simple foolproof instructions on how to find the right position and angle for them.
Trial and error is really the only method left open to you.
Unless you are towing for a large percentage of your driving, it is questionable if such a device will actually save you any money at all. The difference they make is not likely to be more than about 2mpg, and you can buy a lot of fuel for the cost of one of these devices.
You will save more by slowing down by a couple of mph.