What he said.
All depends on the vehicle. If it's in the client's budget, then seamless is the way to go - however, when you're doing fleet work on panel vans, seam it up. Start at the back, and work your way forward. Try not to overlap more than 1".
if it's a car, and total height is less than your printer width, print each side as one single panel.
I disagree with your philosophy on seams or no seams based on the client's budget. It is our philosophy that regardless of the client's budget, we cannot afford to have poor quality work on the streets. In my opinion, seams look tacky and unprofessional... especially seams that could have been avoided.
There is no reason for there to be a vertical seam down the middle of a door panel that is only 48" wide or down the middle of a pickup truck bed side that is only 40" tall. However, I see it done that way all the time.
Of course, some vehicles must have seams. In the case of cargo vans, we try to make the seams as inconspicuous as possible. One horizontal seam at knee level usual does the trick. When the seams are at eye level, they are particularly ugly. When they are down at the bottom of the vehicle, they often go unnoticed.
Try to find the seams on this cargo van we did last week... very inconspicuous.
Even with enclosed trailers and box trucks, we do them seamlessly. The aluminum panels on them are typically 48" or less... perfect for matching up your print panels and producing a seamless job.