When fishing vertical in deep water (30 ft) with a baited circle hook that is in an unattended rod holder, which works best — let the bait lay on bottom with a little slack in the line so the fish can swim off with the bait and hook itself, or reel the bait up a turn or two above bottom and hope he won’t spit the bait when he feels a little resistance?

If your rod isn't tied down, be prepared to possibly lose a rod. I used to use some nylon cord/trotline cord tied in a big loop. And slip one end double half hitch, over the rod handle. And then loop the slack over a dock board. Cheap insurance. And the cord doesn't get in the way, most of the time.

Good suggestion Texas Grown. Would be good insurance. Back to the question though, did you let your bait lay on bottom, or reel it up off the bottom?

I wouldn't let it just sit on the bottom. 6-12" from the bottom ought to do it for eaters. Bigger blues seem to like their bait a bit higher. Say about 2 feet off the bottom.
You've been around here a while; consider how unattended you're talking about. It's possible what you're thinking of could be interpreted as bank polling (I dunno, I'm not the expert on the fine points of the rules). You might want to gear-tag the rig.

Even though my water was only shore line deep to 10 foot max on the end, I did both. My dock was 150'. I sometimes had rods strung all the way to both ends off both sides. But when the fish were biting good, I'd have to narrow it down a bit.

I did have some nights where I had the bait a few off the bottom. And tore them up. Some were straight down off the bottom. Some were with a bobber.

I should have explained that I’m fishing on a floating marina with the water depth being between 30 and 42 feet. Am fishing straight down in boat stalls. The rod holder is firmly secured to a metal post.

I usually first drop it all the way to the bottom and then turn the handle one turn.
If I don't get a bite after a while, I'll turn the handle on the reel another couple of turns keeping count.
If I catch a fish then I try to maintain that depth by counting the turns of the reel.
Same if I'm using a slip float, I'll adjust until something takes the bait and then concentrate on that depth.
Multiple rods makes the task of finding the right depth easier.
I try to use a weight that is just heavy enough to keep the bait down, and so the fish doesn't turn loose and a split shot usually works pretty well in deep water, and just takes a little longer to get down and having to help feed the line out by hand instead of the weight taking it straight down.

I do this type of fishing when I go fish Lake Bridgeport. There are no rod holders so i have to tie my rod down. But back to the question. I have done both. all the way down and a few feet off the bottom. I have noticed a few things. Off the bottom has produced a majority of my fish. I also no longer put my bait alerts on. I lock the reel and loosen the drag with the bait alert on. This has worked the best. Fish hook themselves and the bigger fish will set off the bait alert.

I like Sirjet's description have to give that a try, myself.