had a couple of chain drops when going down not-that-rough stuff today, it's tight enough and the chainline is correct, i'm pretty sure (measured) - measured because it came off a couple times when i was pushing hard last week resulting in shredded calves.. moved the cog a couple mm.
it's a cheap narrow cog at the back and a 1/8 chain - might a fatter cog improve things?
if not, can you get like a rear chain guide that sits in the spacers? what would they be called?
it really shouldn't be necessary.. but if it stops the chain coming off it might be worth while.