Time Machine backs up my primary laptop, a MacBook Pro, every morning. I pick when to update it and shut off the automatic updates. I also use some data exclusion options to keep it from backing up movies, TV shows, podcasts (which I manually back up to a different external HD if I care to retain them locally). I make extra backups before running software updates. I let Firefox update without a fresh backup, since I let those updates go automatically. So I could have an issue sometime, but I don't like letting Time Machine make snapshots all day. So yes, I deliberately take the risk of losing part of a day's data sometime. If I care about something then I make an extra Time Machine backup or drag the material to a flash drive on last save before moving on to other work.
I use SuperDuper! to maintain a bootable clone of my primary machine. I make a new one when I upgrade the OS, get a new machine or the calendar year changes. I update the current clone periodically. If I have any doubts, I make a fresh clone instead and retain the other one. I keep notes on any unusual incidents that help me make decisions like that.
I don't back up my MacBook Air. Most of what's on it is derived from the MBP or its archives, and what's not, if I care about it, gets put into Dropbox on the MBA and then dragged out of Dropbox later, onto the MBP. Or sometimes I will use Airdrop or a flash drive.
Periodically I cull data from archives on my primary machine's hard drive. That stuff is offloaded into archives on an external drive.
If I start to wonder about the state of the external drive on which any backup resides, I label it externally with last valid date and get a fresh one to carry on with. I'm struggling with the prospect of migrating stuff to Thunderbolt, haven't bought any of those externals yet, have a ton of Firewire drives from before the days of Time Machine and a sane approach to backups.
