What are safe memory usage limits? The Arduino IDE used to warn you when your sketch used too much memory. This warning now seems to have been removed. I compiled a sketch, and was surprised to see it use 175% of data memory...but not give me any explicit warning that this would be unstable, much less be physically unable to load onto the Arduino.

What are program and data memory usage levels that you should be concerned about? I seem to remember the best practice was to keep data memory under 80%. Is this still true? Is there any general rule for program memory?
The compiler can only report the amount of statically allocated (RAM) memory. Depending on what you are doing in your sketch the value may be important, or it may be completely meaningless.

Suffice it to say, the total of heap + stack + static must not exceed the memory of your MCU.

When it comes to the Flash memory you can go right up to 100%.