Got a 8+ hour rib tattoo at a weight of ~120. Wasn't too bad at all! I mean, more than a tickle obviously, but really, it wasn't like everyone said it was going to be.
It's a giant hawk. Now that I've put my usual 30lbs back on, and because the hawk's stomach is right on my love handle, I joke that now he looks like a chicken (since my love handle makes JUST his body look fatter).
Like mandalinnn, I have a back piece (mine was 10 hours or so) and when they were tattooing over my kidneys, I thought I was actually going to die.
I would get it done when you're at the weight you plan on being.