Whether you carry collision or comprehensive is a measure of your own risk tolerance, risky behavior, and capacity to replace. There's no formula for it. You can pencil it out. As an example, if you have a car worth 5k and a 1000 deductible, you'll get 4000 in value out of your insurance should you total your own car. Over 4 years, that comes out to $80/month. So if you feel you are unlikely to total your car in the next 4 years and your insurance is $80/month for collision, you're better off dropping it. This is a moving target, though - how many cars have you totaled? How many more and how soon do you think you'll total them? Nobody really knows except you, and even then you don't really know that well.

As far as liability limits, that really depends on what you're likely to hit. I carry high limits on mine because I live in a neighborhood where G-Wagens and Teslas are rampant.

I've never filed an auto or home insurance claim in my life, though. Insurance should cover catastrophes, IMO, and anything short of that I expect to come out of my own pocket.