Yet another can of worms. Ask 2 people and you'll get 3 answers.
I have tried using skunk scent, coyote urine, racoon urine and combinations of all 3 and using nothing at all.
After 30 plus years of predator hunting, I can't prove or disprove the effectiveness.
In Sceery's book he recommends filling fuel tank(s) on your vehicle the day before and NOT in your hunting boots. This makes good sense to me and I always try to do it this way. If I have to fuel the morning of, I wear different boots and leave my hunting boots and coveralls in the truck while fueling. Your boots will pickup odors off the parking lot at the pump and fumes will penetrate your clothing. I avoid this at all costs.
But then I smoke in my truck while in my camo and the reality is that is probably just as bad. I figure eliminating the fuel odor is the least I can do. At least until I get the guts to quit smoking.
I had a some point a bottle of pure skunk juice. OMG you didn't even have to open it to smell it. I stored the jar inside a larger jar and you could still smell it on the bench when you walked by. I used it while predator hunting sometimes and have no doubt that it covered my scent. All I did was set it on the ground (down wind) and just loosened the lid.
One stand that comes to mind I had the bottle sitting out as above and the coyote came charging in all the way to 50 yards or less. At that point he slammed on his brakes turned 180 degrees in mid air and by the time I got the gun on him he was moving at least 60MPH and made it all the way to 125 yards before the bullet caught up with him.
I don't know if on his approach he ran into that wall of skunk scent or he spotted me but either way having the scent there and opened didn't appear to hurt anything.
Finally gave up and got rid of the skunk scent. There was no place in my county that I could store it and not smell it. But for a while, I used it often. Still can't prove or disprove it's effectiveness. I don't think it hurt anything to have it there.
As I said under the "Stand" section, I've seen too many coyotes come charging in ignoring wind and scent to say that a cover scent is mandatory. However, I have called a lot of stands and never seen a thing. It's possible that I could have been winded on some of those stands so using a cover scent all the time might be a good idea.
I have used cover scent while deer hunting and can vouch for it's effectiveness. I've had deer walk up on me from downwind and have no clue I'm there. I'm not calling them just sitting there blending with the terrain. Because of my experiences while deer hunting I believe that clean "scent free" clothing and clean "scent free" boots are critical for both deer hunting and predator hunting.
I don't think you have to use a cover scent but it's probably best if you do. Keep it simple and quick. The less time you spend messing around at your stand the better. Get in, get settled and be prepared to shoot.
A lot of times I will carry a small spray bottle in my pocket or bag. It's fast and painless to just punch the sprayer a couple of times on the downwind side of your stand. Let some of it hit the tree or brush next to you and the rest can float downwind.
Which scent to use is up to you but it makes sense that it be something familiar to predators. Coyote, fox or racoon urine and if you can stand it, skunk scent.