Prevent light leaks
If you design adjoining colors, it's standard practice to trap one with the other. Otherwise, as the vinyl shrinks you will have unsightly light leaks.
Regarding traps, I think most people try to make them as small as possible. But I don't think it's critical. I have let faces go out the door that I thought had traps that were way to big, but once the sign was up it looked fine. It looks far worse to try to cut the trap too skinny and end up with leaks.
Also, I have always in the past made a great effort to trap light colors with dark colors, but if you do it the other way around it is often not objectionable-looking. I have seen traps that I thought looked horrible up close in the shop, but once the sign was in place it was hardly noticeable. For example, I used to go out of my way to make sure a black outline was on top of a colored letter, rather than the colored letter on top, even though it's harder to register. I don't worry so much about it anymore. From the ground it usually looks fine.
Of course, if your design has no adjoining colors at all then you avoid the problem of traps and leaks. I am assuming you are lettering white or clear faces with first surface translucent vinyl. If you are back spraying all colors, second surface onto clear plastic, it's especially important to keep the design simple, if at all possible. Adding outlines and shades, or having translucent colors butt each other, can turn into far more work than you expect because of all the masking and re-masking necessary.
Applying a digital print, of course, is easy, though fading is always an issue.
If a cabinet requires pan faces, avoid using flat plastic. The mark of a well done lighted sign is even light distribution. Hot spots and shadows spells "cheapskate," in my opinion.
Brad in Kansas City