Attacking Helpless Targets

Hi all, a couple of sessions ago we encountered a group of goblins who were sleeping in a gorge. Because they were considered 'helpless' we were not sure if we could coup de gras them from range or what the rules were in regards to that (DM went with, auto hit if you wanted to roll damage, or roll to hit for coup de gras) and we were intending to find out afterwards. However I think it seemed to slip our minds :D

So what are the rules regarding coup de gras at longer range? and a related question, how does non-lethal damage work with helpless targets?

Paraphrased, the applicable rules say: 

1) Any helpless target (sleeping, paralyzed, etc) is automatically hit, and that if the attacker is not engaged then the target can be automatically slain. Otherwise the attacker does normal damage.

2) Nonlethal damage is subtracted from a target's HP just like normal damage, but is less likely to result in a fatal wound once the target drops to 0 hit points or fewer.

This seems like an area where the GM needs to make a call based on the situation.  I think the spirit of the rules is to allow a coup-de-gras when you're not distracted and have the time to make a clean shot.  As a general guideline in my game, I'd say that a player could automatically slay a helpless target with a ranged weapon unless there were any penalties to hit for a normal attack (farther than short range, lighting, cover, environmental factors, etc). In those cases they would roll damage normally instead.