Personally I prefer to let my cats have indoor/outdoor access through a catflap. (I'm saying this even though one of my six has been AWOL for 3 days, but at the moment, I'm not overly worried).
However, before you allow your cat to go out, please buy yourself a decent book about cat care. If your cat went out recently, she coult already be pregnant. My advice would be to tell the vet to continue with the spaying even if she's in the early stages of pregnancy, but I'm pragmatic about this: it's hard to home kittens.
Before you let her out:
Get her spayed.
Have her vaccinations done.
Get her microchipped and put a snap-free collar on her.
Teach her to run to you when you call her. You do this by sitting next to her, stroking her while softly saying her name over and over. Also, when she's hungry, rattle some munchies (dry cat food) while calling her, when she comes to you, give her a few munchies. After you've done this over a week or so, start standing by her dish, call her name, but don't rattle the munchies. Give her a few munchies when she turns up. Soon she'll just answer to the sound of her name being called. (Pavlov did the same trick with dogs and bells!
) Then you'll have some reassurance that when you let her out she'll run home at the sound of her name. Don't chase her though, she'll run away.
Cats don't head for the hills when you let them out, they hang around where they know food, shelter and affection are. They'll only go missing if they are frightened away, run over, or drawn by other attractions (unneutered toms looking for 'love'; or neighbours who feed them food they prefer).
You need to be realistic about the risk of the roads. If you have a garden which allows easy access to the road, such as a semi or detatched, then your cat would be more likely to venture there than if you have a terraced house which provides a definite distinction between front and back. Always give your cat hell if she follows you to the front door.
Whether you decide to let her out or keep her in, please learn more about cat care. Indoor cats are still at risk of fleas, enteritis, flu, FLV, FIV and getting pregnant. A determined female in heat will get out and get herself a bellyful of kittens
Unmated heats lead to ovarian cancer.
If she were my cat, I'd let her out. A lifetime of litter trays!
BTW I think Victoria, the AWOL cat is visiting neighbours.