Spiders are actually good to have around your plant since they eat problematic bugs. I never kill spiders I find in my house. I let them set up shop and do their thing.
If your only seeing some damage on a few leaves, then the few little hoppers aren't doing much harm, it's when they swarm is when they become a problem. They do eat leaves but tend to leave the flowers alone. What you really want to look out for is mites, aphids and tunnelers that dig into your leaves and eat it from the inside out. Though a healthy plant tends to resist harmful pests rather well.
Now to answer two questions at once:
The potency of a plant is determined by the % of THC and other cannabinoids in the heads of the crystals that cover the buds and parts of the stalks and leaves. What I have experienced with early bud is that it gives me a quick buzzy high, since it's all pretty much a low concentration THC in there. If you used a microscope to take a look, you would see those crystals are small and clear. The other ingredients in a good stone is CBN and CBD, which are in charge of that "couch-lock" effect. Those are produced as THC breaks down and the crystals start becoming cloudy and amber in color. If you wait too long to harvest, all the THC breaks down and you'll just feel tired with not much high at all.
The perfect time to harvest is up to your preference.
The mature crystals are called capitate stalked glandular trichomes.
Here is a good thread that explains the difference rather well.
http://www.thcfarmer.com/forums/f34/harvesting-your-trichomes-36014/
Though you will need a little lighted microscope to see them. I got mine at radio shack for $12. Anyone I asked didn't know what I was talking about, so I just asked where the magnifying glasses were. The lighted microscope was right next to it. It's a 60x-100x magnification scope.
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You can also go by the hair color. If they're pretty much all turned color and have started pulling back into the bud then its time to chop. Though I find this method hit or miss, as each strain is so different.
I really dig your waste not attitude