I think the time to be agnostic is when you have evidence on both sides of a claim. For example, I'm agnostic about the existence of a historical Jesus. I think a reasonable case can be made that Jesus was a man who was turned into a legend over a period of a couple of decades. But I also I think Dr. Carrier makes a great argument that Jesus could have started out as a celestial deity who was then euhemerized by the early Christians (much like Romulus was by the Romans). Having no way to rule out either of these plausible positions - I'm agnostic.
But when you have no good evidence for a proposition I think "atheism" is the most reasonable position. For example, I'm not "agnostic" about the existence of flying unicorns. I believe they don't exist. As I believe bigfoot doesn't exist. As I believe that aliens had no helping hand in building the pyramids.
Of course I can't prove a negative. I can't "prove" flying unicorns don't exist, or that bigfoot doesn't exist, or that aliens didn't help build the pyramids. But it's not necessary for me to disprove their existence to be rationally justified in my disbelief. Because the default position when addressing any claim is disbelief. It takes some prior experience or knowledge or new evidence to move us from disbelief to belief.
The absolute lowest bar for agnosticism is having some way of establishing a claim is possible - either through president or something of parallel comparison. For a God or Gods, it has not been established it's possible that they can exist. Much less plausible that they do exist. Thus atheism.
Or, to put all this into one pithy little soundbite; having no good reason to believe something is at least one good reason not to believe it.