Shortly I will be leaving for about three weeks and my guitars will be staying at home. I live in the southeast and I'm wondering how my guitars will be affected if I leave the AC off. I probably would just leave the A/C on around 85. At this point in the year it will most definitely get up to 85 in the house but may come down a bit at night. I understand that guitars are generally comfortable as humans are comfortable, but am I overreacting thinking they will get messed up in temperatures ranging from probably 75 to 85 degrees for three weeks? This is the kind of stuff I never gave two thoughts when I was young but...I've invested a good amount in my guitars since then!
They'll be totally fine. Really cold weather is what you have to worry about with guitars, warm weather bordering on really hot doesn't do much to them except make them prone to be sweat upon.
I'd be more worried about humidity than temp if you have nice acoustics. Temp swings will detune them but they are easy enough to retune when you get home.

Is keeping the A/C at a lower temp really an issue if you are protecting an investment?
Yea I don't think the heat will affect them. I don't have AC and I'm pretry sure my apartment hits 85 degrees at times.. the cold does effect them more.. My buddy's acoustic split right on the face of the guitar from camping outside with temperatures that were still above freezing.
It's the humidity not necessarily the temp. Depends on what type of climate you are in. If it's in the regular range of 40-60% you should be ok.

Unless you're in the desert, mountains, or in a humid hell pit like the Gulf Coast area you are probably in that range.