Do you let guest dogs off leash?

Never. A few times I sat for a neighbor's dog, a very smart schnauzer who was allowed to walk home off leash from a playlot with her owner. Despite knowing this, I would never even think of letting another person's dog off leash. I can't imagine why any owner wouldn't appreciate your good judgment. Whatever level of recall an owner thinks his dog has isn't a reliable indicator of how the dog will behave with another person. I will only allow dogs off leash in my fenced yard. Even then, I was kind of afraid that a tiny little rat terrier might dig under the fence or squeeze through an opening on the quest for prey (I live in an urban area and there is definitely prey, which this rat terrier immediately recognized. As a result, I took the dog into the yard on a leash for his pottying.

I agree with all the other posters - NEVER, NADA, NYET! :)

I have a few owners who bring their dog to me trotting happily behind them with no leash. Remember, you aren't the owner and the dog may respond differently to you. Like another poster said, I too have a fully fenced back yard, however have brought some of my more skittish guests out on leashes at first because I didn't know how they'd react once they got out there.

Remember, even if the dog is perfect and does listen, what happens when someone's aggressive dog gets out of the yard and the guest dog is just off leash minding his or her own business? You absolutely don't want to throw yourself in the middle of a dog fight.

It sounds like you are doing the right thing - maybe instead of volunteering the information that you do intend to leash them (even on a long leash), just noncommittally nod and compliment them on their recall skills!

Never! And I frequently care for some dogs in families that never leash their small dogs inside their gated, fenced, secured community. That said, I know of a couple instances where dogs (not mine nor any in my care) squeezed out underneath the fence while with their parent. The temptations for a dog to explore new scents, chase squirrels, etc is too great.
I always tell them before any services start that while walking their dog unleashed may work for them as the parent, I will not provide walking services for a dog that cannot be leashed. All dogs in my care are always walked on leash.

Absolutely not. Never, never, never would I take a leash off a dog unless....they are in my fenced backyard. I think you answered your own question. You already had the experience of letting a dog off leash you thought you could trust and there you go, he went chasing after something. Don't trust any dog. I know a sitter who did that and the dog ran away. The more she chased it the faster he ran. I learned from her experience.

Never. Never. Never.

Additionally, every dog wears an extra collar for the duration of the stay that says "Help! I'm lost! Call my dog sitter, Carmen, at xxx-xxx-xxxx." I write the From/To days on the back with a Sharpie and put a piece of Scotch tape over it; it comes clean with rubbing alcohol.

Added layers of security can't hurt!