They were stored vertically in my basement – so a constant temperature year round. They didn’t leak stored like that. I only noticed it when I brought them upstairs to change the water out, one fell over and lost about a tablespoon out of the main opening in less than a minute. So i put the other two on their sides as well to see what they would do, and both of them also leaked copiously from the main opening. They didn’t do that when they were new. I’m still using them, but I take more care now to ensure they always stay upright.
One note about the Rhino cans – I just changed the water out after one year of storage. I have 3 of the Rhinos, and all three have bulged out quite a bit on the side that has the indentations for stacking (not so much on the side that has the interlocking projections). If laid on the side all three started leaking significantly, so much that you wouldn’t want to store them horizontally for more than a minute or so. Additionally, the bulging prevents them from being stacked at this point – they are simply too deformed to fit together anymore. I didn’t leave them empty long enough to see if they would return to the original shape eventually. No issues with the water inside when I changed it – they seem fine as long as they are always held vertically.