When to pull a pork butt...

If you can probe it all over with no resistance I'd pull it. However I'm by no means an expert.

Seems early but...how does it feel when you probe it...like buttah or still resistance.  If it feels right then time to declare victory-if not then let it ride.  Pork butts are very forgiving.
I generally see them finish around the low 200's but every pig to his own.

very difficult to dry out a pork butt   I'd let it get to at least 195  I normally pull mine between 195 and 200

You can pull now if the bone will pull. There maybe some areas that don't pull the best, so just chop that. 

If you have to FTC, you're likely okay to pull now and pull all of it. 

I pull my butts at 175° anyway,  but I chop. 

I pull at 200. The stall at 185 is normal

When I did one yesterday; see http://eggheadforum.com/discussion/1195693/pork-butt-cook-in-the-style-of-500/p1
Mrs. 500
was pressing me to finish she it so we could eat. Against my best effort to keep this cook a true low and slow, I bumped the Egg temp up to about 375 after it had reached 185 IT. I probed it and it was soft in some places and some resistance in others. It was like 200 in some spots and 188 in others. I pulled it and just before I wrapped it I pulled off the money muscle and ate it immediately. That was very tender and tasty. No sauce needed. Again against my will I unwrapped it after a 1 1/2 hour FTC. I wanted to keep it FTC for up to 4 hours as I've read a long rest is s good. Before I pulled it I asked the eaters to have their plates ready as I felt this would dry out quickly after pulling. Parts pulled easily and other had some effort to tear the strands. Mrs. 500 said it was tough and dry like I suspected it would be because of her rushing me. (She didn't want our guests to wait because they had to take their daughter to a social event later that evening.)  I thought it was not my best effort as well. I need to adjust my start time earlier so I can pull it off when I want and FTC as long as I want next time. I may even F and put it back on after the stall next time. We'll see. Was fun experimenting and gaining knowledge for the next one. 

I think a great zone to be in is 195-200.   I did shoulder yesterday and took it off the egg at 197, let it rest for ten mins and it was so easy to shred into pure bbq bliss i was, as i always am, ultra pleased with the bge results.   I am sure other smokers yield great results too but bge with guru set up and the lack of attention required is just awesome.