I will add myself to the list of those that clear over the decals.
I'm with MonoPed and Scuderia on the clear adding durability to your decals. I've got cars over 20 years old that I've reccently polished again, without any incidents regarding decals, and they look good with the rest of the cars in the case.
The decals they use on the real cars are 3mm thick on average, kinda hard to replicate in scale. My other opinion is, if you want ultra realism by having the decals not cleared, you have to cut out every letter, number and logo, because there is no clear film around any of the graphics on the real cars.
Of course, there isn't a single contest judge here in the midwest that will pick a car over another because the decals aren't cleared.
Do not consider my opinion a slam on anybody's opinion by any means! I have my opinion that I'm happy with, and won't judge others on a different choice. Shoot, this question is almost as debated as "What is correct for Petty blue!