Props are designed to be efficient in one direction. And they usually put the size markings on the front of the prop. So the size numbers should pretty much always face the direction your plane travels. I say pretty much because there is always an odd ball/rebel out there who doesn't follow the "rules". Lol Now, you can run the prop backwards, and it'll still produce thrust, but it'll be louder, less efficient, and produce less thrust.
So, the moral of this story is to make sure the size markings on your prop faces the direction your plane flys, and make sure your prop nuts are good and snugged up. Use nylon locking nuts or if you have one of those mini spinner style nuts, use a scrap piece of landing gear wire or something to stick through the tiny hole and snug it up nice and good. I do this and have never had a prop come off. And I'm sure I run a number of my motors the "wrong" way according to it's thread pattern.
Keep note tho, on tractor style planes, especially on minis, the motor pod/mount might have thrust angle build into them to counter act the torque roll the motor produces. If you use a motor going clockwise on a pod that has right hand thrust angle built into it, you'll have torque roll issues.