Is it necessary to keep a truck spotless? Not in my opinion, however, a clean truck is a happy truck. When a truck is happy it will not have as many maintenance issues. Why do I think that? In my twenty years of being a mechanic, the trucks that were kept clean were the trucks I had to work on the least.
This includes the interior; electrical issues can be caused by dust. Switches, gauges, and connections are, for the most part, sealed. Switches have moving parts, dust can cause wear, and that will allow dust into the contact point area causing a failure. The same for a gauge, the dust has a harder time getting in, but can still cause a failure. Connections, whether it's a plug in or a screw fastener, could in time, due to vibration, become loose enough for dust to create an insulating layer and cause a failure. These connection failures are hard to detect because the tech will have to unplug or unscrew the connection to do a test. This in essence will " clean" that connection solving the problem.
When you keep the exterior clear there are several things to consider. When washing the truck, water running down the sides, rinse the frame and the "not so shiny" things. This may not sound to important, but getting the road grime and chemicals off the underside can save you time and money. Grit and grime get into the tight spaces and cause premature failure of many underside items, like u-joints, s cams, slack adjusters, seals, and suspension components.
A clean truck is less likely to be pulled in for an inspection, theory is, if you keep it clean you probably keep your maintenance done properly. I heard this from several DOT inspectors, there is a however to this, if they need to fill a quota for the day, they choose the clean shiny trucks for a quick passing inspection.
There is a bit of pride in driving a clean, shiny truck, even if you don't own it. The customers like clean equipment also, if you care for your equipment like that you will take care of their freight just as well.
Take care, keep it clean.