Having done the 4.0 headswap on my '83, I just went through this. I left the canister unconnected on the first go-round, and although the engine worked fine, I did often have the lingering and not so subtle aroma of raw gasoline billowing around the Jeep whenever it was parked. On hot days after a fillup, as the cold fuel fresh from the underground tank warmed up in my gas tank, I would sometimes even find fuel spots on the ground.
The plumbing is a little simpler for the MPFI setup, so I went back to the junkyard and pulled the cannister and plumbing from the '91 Wrangler donor. After hooking it up, all those fumes go into the intake instead of the atmosphere, and all the gas I pay for gets burned. No more gasoline stench, and no more dripping fuel. Don't have to worry about pressure differential between my gas tank and ambient. MPG went up a tad. I'm not a big environmentalist by any means, but it's nice to know I'm not polluting when I don't have to be.
My advice is, hook it up.
"We are all of us in the gutter...
Some of us are looking at the stars."
~ Oscar Wilde via Chrissie Hynde