Is sexing my plants before time bad??

I had a bad experience once when I grew a few plants and I guess I didnt catch the boys on time and they ruined all my plants...I wasted a lot of time and money..
With that said I started growing again and I have 7 plants in 5 gallon pots at about 18 inches tall...
Do you guys think its too early to put my plants to flower?? I know people say you cab flower whenever you want, but would I get less yield ifI do??
Or how about if I flower them for say a week snatch the boys and re-veg the females?? would that stress them out?

Put them on 12/12, find the boys, kill'em and then back to veg if you want or stay on 12/12. They will be fine.

yea man, what you can do is turn 12/12 on for a week and let them show their sex. Once every plant has showed their sex, just give the axe to the males, and go back to your regular veg cycle.

It wont stress my plants at all if I do that?? they are still only about 18 Inches..

Your plants will be fine. You'll lose about 2 weeks of veg time, but at least your males will be gone.

If you are that concerned about stress, number all of your plants. Take cuttings and place them in numbered cups and place the clones in 12/12 any that show male, the plant it came from is male.

I rather lose 2 weeks than 2 months honestly...thanx for the advice..
The problem with taking the cuttings is that I only have 1 light so I wouldnt be able to have some at 18/6 and some at 12/12

The above is certainly not bad advice.

Are you running out of space, or do you want to start them flowering now? Depending on the space you have 18" might be a good time to flower, remember it could grow 2-3 times its size in flower. Are you not able to check it often enough to pull males? Were you unsure on how to sex and missed the males before?
If it's not a space issue I see no reason why you can't just grow them till you want to flower, and just watch them to find and pull males. This is how I do it, and there really is no mistaking a male, if you know what to look for you should be able to pull it long before it releases pollen. In fact, most of my males will sprout balls in veg before my females are ready to flower, and it's not uncommon for males to flower under 16+ hours of light (many males flower for me at 18 hours).

If there is a compelling reason to sex early, in addition to flowering the plant then reverting to a veg, you can also clone them, flower the clones and sex. Clones themselves set to 12/12 after roots can yield a surprising amount of bud as well

thanks... The problem is that I didnt catch the males on time even though I have them in my house... I can see them 24/7 if I want but I guess I missed it...When I realized balls all 5 plants had balls...
I want to flower now because I dont want the same thing to ha[[en and me waste all my time once again :/