I have a couple of thoughts on that, transparent for things you do not mind letting others see, like lets say shampoo, sun screen, toys for kids, swim gear, etc. But for thinks like cameras, video recorder, portable DVD player etc, opaque. I will go all opaque as I already have them, but if I had to buy new, then I would get some of each or carefully pack those expensive items so they can not be seen, especially if not packing in a trunk, but the back of a van. Unseen items are not at tempting as those easily seen. We also tend to put the items that are the most valuable in unlikely places.
Just overly cautious I guess.