when you have 20 rentals, when you adjust a coverage item, you essentially are running your own insurance. Assuming you can save $300 each, that is $6,000 saving a year. Compare with $70,000 in coverage limit drop, it takes 11 years to break even, using static analysis, assuming only one hit. How good is the chance for one of your rentals to completely burn down in 11 years? No one can really tell. But I would say the chance is very small.
There is chance for fire damage, but most incidents are not a complete burndown- it wont even hit your 230k limit,