On a single color vehicle, it is 100% necessary in my opinion, you need something to break the color up. On a two-tone rig, it helps accentuate the colors if the correct color/colors are chosen.
All of my rigs end up with a pinstripe. It's an art for sure, you have to have an eye for the body lines. I honestly don't know how many I've done at this point, but I've laid out a lot of vinyl tape!
Some examples:
My old '98 Z71 ECSB
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My Z28
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My old Vortec Max Sierra
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My Tundra
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And of course the stripes on my K2500 Suburban
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