Depends on what type of grass really
like zoysia, you don't wanna cut it any lower than the 2nd and 3rd wheel settings on a standard 9 setting mower. weeds it doesn't matter really, I cut it on the lowest setting and it's still a green carpet.
for a good looking lawn, whatever setting makes it look good without causing it to brown out by being short is ok, except fescue and grasses like this, they do better when cut higher. but if your yard is like mine (weeds) I hate it getting out of hand so I keep it cut as low as I can, the sun would save me some trouble if it would just burn it