Here's how I'd approach it strictly from the financial point of view. For sake of argument let's say the truck costs $25,000.
How much interest could you earn on an initial $25,000 deposit over the course of 5 years, putting the money into safe cash equivalents that would allow you to pull out the $417 a month needed to make the payments? Perhaps a CD ladder could work with a lot of it with $5000 of it "maturing" each year to make the next year's payments.
Unless the interest I could safely earn (no risk of principal loss) by taking the financing "beat" the $2,000 cash back by a considerable margin, I'd just pay cash and not have the debt hanging over me or the added complications of having another bill to pay.
If I could make (say) $3,500 in interest -- unlikely in this pathetic yield environment -- by financing it I'd finance it. If it were more like $2,300, that extra $300 for financing isn't really worth taking out a loan and having debt, not to me. And if less than $2,000, it's a no-brainer; pay cash all the way.
Anyone who has the ability to pay cash but takes 0% financing (I've done it before on a new HVAC system, there was no cash rebate option) should, IMO, *absolutely* "firewall" that money. It's spent. Know that and resist the temptation to spend it again. $25,000 of that (using my example) is already spent; you're just allowed to keep the "float" on it until you have to pay it back.
I didn't approach the "I despise debt" point of view which would say to just pay the cash and don't play with fire, but if you're extremely debt-averse, there's nothing wrong with making a (possibly) suboptimal financial decision of paying cash up front if it makes one feel more in control of their financial picture. (Then again, if you were one of those, you wouldn't need to ask this question...)