We all deal with a lot of tension in our lives. But for our guitars, the tension is never-ending. Your guitar's neck can easily have 150 pounds of tension pulling on it at all times. Now imagine that 150 pounds trying to rip the very thin guitar top right off of the instrument! Sometimes I'm amazed that all of our guitars don't spontaneously implode upon themselves.
Three things are going to affect string tension. This isn't rocket science, but it's worth a quick review.
1) String Gauge: Simply put, thicker strings are going to have more tension.
2) Scale Length: Scale length is the distance from a guitar's bridge to the nut. When tuned to the same pitch, strings of equal gauge will increase in tension as scale length increases.
3) Pitch: String tension increases as pitch is raised. (See? Not rocket science)
Nowadays, many players routinely tune their guitar to alternate tunings. Slide players have been tuning to an open chord for years. Joni Mitchell has used over 50 tunings during her career. Lots of metal players drop down to "D" and lower. All of these tunings are great; they open our ears to new sounds and put our fingers into new positions. But they can cause unwanted issues with your guitar's stability and tone.
There's a very useful website, www.mcdonaldstrings.com/stringxxiii.html, where you can enter the gauge, scale length and note for each string on your guitar. It will allow you to see each individual string's tension. Using their calculator can help you design a set of strings customized to your preferences and tunings.
I've got a great friend; to protect the innocent we'll call him "Z" (You know who you are). "Z" plays a stunning 1966 Rickenbacker 360, but uses a completely unique tuning, GBDGBD low-to-high. Well, his Rick wouldn't play in tune, the truss rod was always needing adjustment, and frankly it sounded like hell. I sent him to the string calculator site, where he designed a set of strings specially balanced for his tuning. The difference was immediate and profound. Suddenly, that old Rick had the chime, balance and presence that makes it so special.
The moral of the story: give those alternate tunings the "a-tension" they need.