Are your cats even wanting or trying to get outside? It doesn't really sound like they are. As for me, if my cats weren't trying I wouldn't even think of it.
I have one that always wants outside and so I thought if I just watched him, that sort of thing....it was a disaster as the more comfortable he got the farther he went, he adores trees so he would take off and up the trees just like that (actually our adventures would make a fine kitty comic
)He would just take off like a jackrabbit and then one night I lost him....we found him the next night but only because a neighbor saw him. He had no thoughts of coming back home at that point. We did the harness thing too but that just made him want outside more. He is a total escape artist so the problem comes when we open the door. We have to be vigilant or he is gone. It's better in the winter....it's spring now so we'll see how this year goes. I wish I could let him be an outdoor cat as he really does love it but we live in a very busy suburb, next to a highschool and highways are just a hop, skip and a jump away. If we lived in the country I would let him be outdoors.
There are just are too many dangers. I'm not telling you what to do, each cat and neighborhood are different BUT unless your cat is making it an issue, I guess I wouldn't invite possible trouble.