Mike - I've done lots of boneless butts from Costco. I almost always tie them, though I've done both.
The only reason, really, to tie them is to try and keep the butchered hunks of meat in a cohesive mass - and to minimize small chunks wandering off on their own getting dried out. So, when you tie it, do it just enough to get it to stay together. Don't worry too much about it - it's not like trussing a turkey or tying a butterflied leg of lamb. The reason you don't want too much string is because when it is done, when you remove the string, bark inevitibly comes off of it too.
Last, before you tie it up, I like to salt and pepper the inside bits of the butt that will be tied up (i.e., the parts that won't be exposed to the outside smoke). It's a way to boost flavor a bit. However, I only rub the outside of the butt, since sometimes if you rub the inside then tie it, it gets a funky gray-ish color. So... salt and pepper the inside, then rub the outside after you tie it like normal.
Enjoy!