If you want it to be potent, keep them green and as healthy as possible, with as much light as possible(when you reach 1000 watts per plant, that's enough.lol)(seriously).
Yep, it's very possible to take a unknown bagseed and turn it into some really good smoke,
much better than the bag of smoke that it came from. On my first grow i got my seeds from a buddy who had these seperate because it was better than average weed, for our area(Mexican Dirtweed.lol). After I took 'em and grew my own, I let him try it and he almost shit. He was amazed at the difference care can make. It was twice as good as the original stuff. I was amazed too, actually, because it was my first time growing, and i never saw weed like that before. Hell, I didn't even want to cut 'em down.lol Anyway, long story short....Yep, can can take seeds from dirtweed and turn it into some pretty decent stuff, with a little care.
BUT, I was the first one to ever have a "real strain" in my town, and it was Crystal, nice white and crystal covered, *drools* and that bagseed couldn't even come close to that, even though it was decent. That's when I was truly convinced that genetics are absolutely necessary if you want the best of the best. That was the first time me or any of my friends have even seen or tried, any top level strains.(never once, in the 15 years that I've smoked, have we come across any truly good weed) When we tried it we were all floored, literally.lol And now we're all spoiled, now that i have a new hobby.