I've only done a few butts, but I don't ever trim off the cap and have what I would consider great results. I may trim it up a tiny bit if there's loose stuff, but it's more for presentation than anything, I guess. Don't think it actually has any impact on the finished product!
Thats the fat cap, yall wanna keep it! That be a huge pocket a flavor an yer automatic marinator while smokin. It will slowly melt away while cookin an run down the meat keepin it moist. When yer all finished with the smoke ya can take whats left off, I never have, just mix it in with the rest a the meat. Never much left on mine anywho's.