We are there June 13-17 and I was kind of thinking aiming for Fantasmic on Wed June 16 would work. It is grad night, so just one Fantasmic, and a bit earlier. I thought maybe not as many people would hang around for it that night. Am I wrong? We hope to catch the fireworks from Main Street earlier in the week, but thought we could than hang out and watch the fireworks after Fantasmic.
Any thoughts on this plan? Am I wrong to think it is not going to be as crowded?
Great question; I'm not sure anyone KNOWS, but that won't stop people like myself from making our best guesses.
This is all assuming the weather is favorable for Fantasmic! on all nights (if it's pouring rain they won't play it and if it's very lightly raining then even if they do play it, it might not be fun

).
I heard reports that when fireworks are offered on Grad Nite days in previous years, the show is actually super-crowded, probably because it's the only show to do in the park.
There are at least two differences between your situation and those past situations:
*Fantasmic! is also showing (it's not showing during most Grad Nites)--this means there are TWO things to see late at night
and
*In theory, AP-holder visitors may be wanting to visit to see World of Color, which of course will not be playing on a Grad Nite (in other words, MAYBE lighter crowds for Grad Nites)
So, in short...I don't know.

I'm not sure it will be any less crowded--in fact, the showing of Fantasmic! hints to me that Disney just MIGHT be expecting higher crowds though I think it's more likely that it's just compensation to guests for having two Grad Nites in a row. Regardless, at worst, it could be that there's less camping out than usual, which would mean you could actually get a decent view of the early show without spending most of your afternoon there.
IF there's any way you can do the second show of Fantasmic! instead on another night, then I'd recommend that, because you can absolutely get good seats with about an hour wait, while the Grad Nite show is an unknown. Otherwise, I think the Grad Nite show might work OK, though if that's your last night I'd personally kind of want to do it earlier to make sure I got to see it. (If it's not your last night, or if you wouldn't be crushed to miss it, then 6/16 is fine; you'll most likely see the show if you make sure to camp out when you need to, crowds or not.)
Have a great trip!