This has probably been covered once or twice before but ....

* For day to day storage, is it better to keep guitars in a case or in a stand?
* Is there a different answer for acoustic and electric?

- Note: I'm not worried about then being knocked over. My concern is the temperature changes, humidity, etc...

So long as it isn't in a greenhouse, or leaning against a radiator, it should be fine on a stand.
A case is better but a stand should be ok. A case prevents against knocks, small spillages, rapid changes in temperature etc. Also, some stands eat away at nitro finishes...
a case is better imo, it's offers more protection from other factors.
Ibanez RG321MH (Air Classic/Tone Zone)
Fernandes Telecaster (Twang King/stock bridge pickup)
Blackstar HT-20 (Scumback 55 speaker/ Tung Sol tubes)
TC Electronic Nova Repeater
Lava Cables Clear Connect, Soar and Mini ELC