This video claims this is the only glass cutting method that cuts bottles perfect every time:

"A flaming string is not good. A blow torch is a BAD IDEA. Glass hammer will not work well.
I talked to a few people who have cut 1000's of bottles, they say "you cannot get a perfect cut, top will always be ruined" Not true. Perfect bottle cutting 95% of the time. "

Recycled bottle lamps