Interesting thread! There is such a spectrum of "not literal truth" responses. To me, a lot of things are metaphorical (which the child understands as literal at first), "playing pretend," age-appropriate simplifications, positive spins/being tactful, or even personal beliefs, rather than outright lies.
Adults have to deal with a lot of social nuances, polite fictions, omissions, and saying completely the opposite of what they're actually thinking (or not saying anything at all), in order to function in society; and I think this is something that can be taught to kids starting at a really young age. That we don't tell Billy his new shirt is ugly, for example, even when he asks--we can tell him it looks fine, or that we like a certain aspect of it, or even that we like another shirt better, but blurting out exactly what we think without moderation can be hurtful.
Even something like "monster spray"--the kid is afraid of something, and you are trying to help them not be afraid, using a simple method that they understand. There are lots of other options that could be used, but I can't imagine a child who would be comforted to be told the literal truth, "Actually, there are no monsters under your bed, so there's nothing to be afraid of." When their fears are something imaginary, I think an imaginary solution is a good response, because they will grow out of both ideas at the same time.
I think it would be dodgier to tell kids an imaginary solution to a real problem--like, they're scared of tornadoes, and you tell them this spray will protect them from tornadoes. I suppose it's not that bad if you also tell them concrete, practical things to do when a tornado comes, and teach them ways to deal with fear in general; but definitely just something silly and magical would be wrong, I think.
Also, to me something becomes dodgier if a parent is using a lie to scare their children into behaving--like, the parent has trouble getting the kid to go to bed on time, so the parent says, "If you aren't in bed by x:xx, the monsters will come out and get you!" The world is a scary enough place for kids, I don't think it's right to take advantage of that when there are other options a parent can use.
I feel uncomfortable with lies that seem more about a parent's convenience than their child's needs, or where it seems like the parent is smirking in the background at how "gullible" the child is. That's not respectful, IMO. I can see how occasionally it might be a last resort, a more creative version of, "Because I said so!" but I wouldn't be comfortable with someone who did it on a regular basis.