Straight of Grain for a wall hanging.
 

I'm planning a quilt that will most likely be a wall hanging. It will be about 60x80. It won't be exactly whole cloth but very close. There will be only a few seams. I don't want to end up with a sagging quilt. I do plan to heavily quilt it. I was wondering if it matters if the straight of grain is vertical or horizontal.

My wall hangings have lots of pieces. Also, I have the back be straight of grain lengthwise. I've had no problem with sagging.

The cross grain does stretch more than the length grain.

You can always block your wall-hanging. I do it most of the time
except when I'm lazy. LOL. It hangs a lot better after it's been blocked.

if it's mostly whole cloth I'd put the length of grain so that it hangs. But you say you plan to quilt it pretty heavy so it may not really make a difference.

I think it would be more stable with the straight of grain horizontal, but I would probably hedge my bets by having it horizontal on the front and vertical on the back.

I often use a sturdy backing for a large wall hanging. Something like a decorator fabric to help keep it flat and straight.