Is smoking a boneless pork butt substantially different than smoking a bone in pork butt? Should I tie it up with twine?
The main differences between boneless and bone-in are that with boneless you need to strictly adhere to the 40-140*/4-hr rule for internal temps when prepping and cooking, as it is no longer considered to be intact whole muscle meat. Depending on weight, a bit higher chamber temp than normal may be necessary to pass through this temp range quick enough. A possible down-side is that you may over-shoot the required temp to make this goal and cook the butt too quickly, which usually results in a tougher finished product, not easily pulled, and possibly a bit drier texture. With bone-in, you need only to wait for a couple hours before probing for internal temps to allow the exterior of the meat to be pasteurized. The Food Safety Forums discuss this in detail. Also, the Low & Slow Discussion thread covers many details very well.
Many do tie up boneless meats with butcher's twine so it holds together much better, allowing for more evenly cooked finished product, and in some cases this may help for more evenly distributed moisture content.
All that considered, I personally prefer bone-in due to having the less restrictive cooking guidelines, as well as the bone acting as a shrinkage gauge. Late in the smoke, you can give the bone a light tug and twist to help determine tenderness of the meat. Sometimes temps don't always tell the whole story, so probing for tenderness and a bone tug will give you a solid answer as to if it's ready to wrap and rest or not. When you're ready to pull the meat and bone pulls out clean, you can't do any better for tenderness, IMHO. The bone also adds flavor to the meat...if you can catch the meat drippings (already flavored with some rub on the meat) and de-fat and toss in a bit of cold liquid into the heating pan (water, lemon juice and/or orange juice, worsty sauce, white wine or whatever else trips your trigger) to de-glaze the drip pan, then, add to the pulled pork, you will have all the finishing touches you need for a unique and tasty pulled pork that you won't easily find in most BBQ joints. A little forethought goes a long way towards really great eats.
Hope you weren't waiting around for an answer while your butt was rubbed and sitting in limbo in the fridge...didn't see this thread 'til right now....sorry.