At what point should I switch between a resistor divider and a LDO?
It is never a good idea to power anything from a resistor divider. Any variation on the load will change the voltage, and your MCU will very likely vary its current consumption based on what it is doing at any time. This means your supply voltage will be affected, and as you are wanting to go to 1V8 (the lowest acceptable voltage) then this is an extremely bad idea.

You can get LDOs in very small packages if it is space constraints you are worried about. Getting a fixed version means no external components except capacitors. 

To sum up though, while it is theoretically possible to do it, it just isn't worth the hassle if/when problems start to occur, which is quite likely.