Speaking strictly from a physiological point of view here:
When you use a shin block, the leg should be taken with the upper end of the tibia as this is the thickest part of the thicker bone. This means not the outside but the upper third of the front of your lower leg since on the outside, there is the much weaker fibula bone.
Tibia and fibula can be used together if you don't take it upfront exactly, but slightly to the outer side from the center (where your tibia bone is). In between are the thickest muscles on the front of your lower leg.
These muscles are only engaged if you pull your toes up.
Thus, you should a) never take any low kick with a point of your lower leg low enough to endanger your toes in the first place and b) in an ideal world, your take it with both tibia and fibula plus the muscles in between as cushion. Hence toes up.
The reason why some people favour toes down comes down to technique imho: Muay Thai fighters are light on their front feet and you can use your foot like a spring pushing from the ground to raise your leg faster and get the block where it should be easier in time. That way, you naturally end up with lowered foot. If you raise your leg by means of the hip flexor mainly, this is no necessity.
Now, is there a risk of injury with toes down? Definitely. If you meet a proper low kick with shin against stretched foot, you may well break some bones on the upper side of your foot as there is no cushioning muscle or other tissue and these bones are not made to resist forces from there.
If I had to choose, I'd rather take one or two broken toes if I mess up my block.
