If I think I might want to make changes (except for free same day changes), then I delay checking in until I'm fully decided because sometimes un-checking in can be a hassle.

I also like to check in on the app so I can confirm no random precheck or SSSS situation, which require getting to the airport earlier. On international itineraries I usually stop by the kiosk or desk to print boarding passes anyway for insurance. The last two times I connected through Europe, the United app lost the connecting boarding pass when I landed at the connecting point. At some places this can cause a real headache if there's no desk to print a boarding pass and you need it to get through whatever the next security step is in the process. Even when there is a desk, they may not help - the Senator lounge attendant denied me and told me I could go to a business class lounge for assistance in Frankfurt recently. (Yes, I was entitled to Senator lounge access.)

Not checking in in case you get into a car accident doesn't really do much - even if you're checked in your can still cancel before departure.