You can get away without a relay if the lights are low power. If you want bright lights. Use a relay.
I have had good and bad experiences.
I ran some cheap "low power" back up lights off the backup connector on my boat trailer without a relay. They have work great for eight years. I am stunned they have not crapped out on me. My boat overhangs the trailer at least 6'so backup damage is not an issue. The trailer is used twice a year, 32 miles total, to bring this boat to the hoist and take it home.
In 12 years I have gone through 3 or 4 sets of auxiliary backup lights on my Silverado. None of which lasted more than a few weeks. When they worked I loved them.
I need bright lights as it hard to see out my tinted rear window. My Silverado is 22' long--an extended cab long bed. Turning around means lots of back and forth. If there is snow on the ground I often forget it can be icy and try to plow right through.
I use excellent wiring practices, soldier and heat shrink wrap plus electrical
tape, strain relief, tie wraps and wire loom armor. In most cases, follow on inspections show the wiring looks good. One case the ground lead was broken. Another case the wiring looked perfect but did not work with new lamps. I was in the habit of replacing wire when I replace these lights, however I gave up replacing the extra backup lamps, in frustration, until I develop a more permanent solution.
Why do these fail when my trailer lights work so well? it might be for more than one reason.
Perhaps this is just from backing into snow banks--banging up the low hanging lights or wiring.
One set of tractor lights, mounted upside down, filled up with water. While they were sealed beams the connectors were exposed. I tried drilling weeps holes in the next set. They did not last long either. I found a popped fuse and the lamps not working. These were not bright lamps, just cheap tractor lights.
My conclusion was that bright backup lights need a relay as well as armor. I'll guess my tractor lights were just beat up and don't need a relay. I di need a steel frame with a Lexan cover for backup protection for snow and brush.
I thought the Silverado fog lights, which sell for $35 a pair on eBay, would make good backup lights and are adjustable. I may mount a set of headlights in the back and use the low beams for backing. The pin system for mounting is fast and easy to remove and replace.