Here's a citation for you: if you build a structure that kills the mobs for you, or lets you kill mobs while out of harm's way, that's a no-no. Building a structure that creates mobs for you to fight is a-okay.
Wait, how are you "building" a mob spawner?
If you're curious, my mob spawner will be a 15x15 structure, 20 blocks high, deep underground. Floors will be 3 blocks high, and a staircase of water will flow gently in a helical pattern around the perimeter, safely bringing the mobs down to the brightly lit bottom floor for me to fight. There will be a bit of flowing water on each floor, encouraging the mobs to go out to the staircase. I'm trying to design to balance maximum dirt space with maximum mob flow, and making sure the mobs can't take fall damage, and that the water is never two blocks deep (so the mobs won't drown).
On a side note, with all the ore I'm digging, I regret not doing this while I was still getting XP from mining. Tip to all you pre-spec'd PvP classes: build your spawner now, while it still counts