I've talked to one carrier that said it would be fine if I made my own magnetic sheet cutouts to fit their decals, as long as I was identified when under load. That's the only way I will mark my vehicle. I use it for far too many personal things--it is my daily driver after all--like vancamping in the Nat'l parks, etc. Can't drive a lettered vehicle on some parkways, in some parks, etc.
Something that I have come up against more than once, as well, is that some neighborhoods and businesses (DC area around foreign Embassies comes to mind) will not allow lettered vehicles in their parking lots or driveways. I have family who live in neighborhoods like that, which, if lettered, I would have to park a good ways away if I wanted to visit. No work trucks allowed. Unlettered, just fine.
I have never been stopped in any unmarked van I've owned, honestly, but was stopped in a former express van I once had that was still lettered. Questioned as to why I was out of my area, made to open the back, show my reg, etc.
In my long experience with white work vans going back well over a decade, both lettered and unlettered, and traveling the entire United States in them for most all of that period, I have always been considered professional, polite, and of no threat. Other than the one time in a lettered van.
Additionally, I have been in neighborhoods across the country, and parked in hotel lots and repair lots, that I was told lettered vehicles are targeted for theft far more than unlettered, because of the possible contents.
I know building contractors and carpenters who do not letter their vehicles for just that reason. I also know contractors with lettered vans who have lost thousands of dollars worth of tools and client's property to break ins.
For those reasons, and that it has been far more convenient and practical, not to mention more multi-purpose, I prefer to not have my van marked.
Saying things like how does one know if it is an expediter or a child abductor, even if in jest, only serves to perpetuate a wrongful myth.