I'm with the 'almost irrevocably evil' camp.
If you summon a demon and tell him to kiss a baby, he'll kiss it by biting it's head off.
If you summon a fiendish hawk and tell it to attack and kill the dark king, it'll first perch on his head, then slowly gouge out his eyes, scraping them out with a beak dripping with foul fluids.
If you summon a Balor and say "Hey, mack, how do I kill Dr. Evil!" he'll tell you a way to kill the doc, and that same way will put him in power.
By summoning something evil to do your bidding, you're performing your act in the most evil and dispicable way possible. When there are less wicked options open to you, willfully choosing not to take them and to expressly select to use the most diabolical, cruel, and wicked tools at your disposal is most certainly Evil.
Now, IMC, you could probably get away with having a Neutral demonologist...an 'end justifies the means' kinda guy who doesn't like what he does, but who sees it as the best way to accomplish his goals, or as a reasonable extention of his learning. Such a character would, however, be in continual lure of dropping over to evil, and all it would take would be a carefully-worded suggestion from a demon that he summoned that he actually followed to do it.
....not that being evil is nessecarily a bad thing IMC.
