I am an engineer...which is why i must say no to grinding at all.
It's not so much the grinding that is a problem. If i were to do this myself the correct way to reshape the snout is to use a mill or a lathe or CNC with liquid cooling capability. Grinding generates way too much heat in one particular location which can cause the metal in that area to become brittle. The snout is what basically keeps the rotor on the car.
The reason why i always try to tell people not to grind is for the newbies out there. The 16 year old kids that buy Mustangs and want to run a nice set of 5-lug rims but don't have the cash to do this right and are not very mechanically inclinded. So they read a thread like this and whip out dad's grinder and go to town on the rotor taking off way too much material and getting the snout cherry hot and making the metal brittle. The very idea that this is entirely possible scares me to the point where I just completely wipe that option off the slate when i add my 2 cents in in 5-lug conversion threads.
Not to say that any mod on a car can be potentially dangerous, but there are just some things that can be avoided completely.