I was the one that posted my dogs only wear collars when they're also on leash (not necessarily on walks) - I should amend that a bit, since we do put on the training collar (flat nylon buckle) for training sessions, even if it's off-leash. (The training collar is a signal that we're starting "get serious" time.)
There are a few reasons I don't leave collars on my dogs all the time. First of all, they'd get filthily dirty in two days - I'd spend half my life washing collars! More seriously, though, I know several dogs who have either gotten their collar caught on something (fence, drawer pull, whatever) and choked/almost choked to death. I also know a couple of dogs who, when playing with other dogs, have had the other dog's jaw get caught in their collar. Dogs panic, twist, collar tightens, etc. One died, the other was lucky (the plastic clip on the collar broke). (I also know of a dog whose owner, knowing the danger of leaving a collar on a dog in a run, used a harness instead. The dog tried digging under the fence, caught the harness in the chain link, and died of heat because he was unable to get to to the shade/water.) With 5, it's impossible to supervise them 100% all of the time (and even if I could, these things happen FAST - even if I saw them get caught I might not be able to get them free in time).
My dogs are all microchipped, so they are recoverable (may take a bit longer, but better than them choking to death). I'm not all that worried about them getting out the front door - the only time they'd have the opportunity is when I'm there, so of course I'd be able to go after them right away, and at least have some idea of where they went. And while it is possible that they'd be hit by a car before I could get to them, a collar won't stop that from happening. I do have one who can jump the 6' fence - we keep a close eye on her when she's out, but chicken wire across the top of the fence has stopped her fairly well!
I do put collars on them when they're in the car (they get there on leash, anyhow, so they've already got them on). They are buckled in on their harnesses, so minimal risk of them getting the collar caught on something. In the case of an accident, where the dogs will be frightened, the extra ID is a bonus. When they're in their crates in the car, the collars are attached to the crate.
Those of you who do leave collars on your dogs all the time - PLEASE tell me this does not include when they're in their crate when you're gone! This is one of the most dangerous times to have a collar on - especially one with tags. The dog can by laying in the crate, close to the side, the collar gets caught in the wires, the dog tries to sit up, can't, panics, chokes, dies.
It is, like crating, a comfort-level thing. I'd rather work with the obscure risk that they'll get out and get lost than the more likely one that they'll get their collars caught on something.
Julie