Question: I've seen a lot of different topics and suggestions on aligning and inputting buttons/text, but the ways I've seen seem kind of risky.

What is the optimal way, for example, to add two buttons, stack them together, and have them be 10% from the bottom of the screen, and centered horizontally on all devices?

Answer: 
The optimal way would be using storyboard for implementation and use of constraints in a proper way.
For example, as you suggested you want bottom space to be 10% of device height so it can be done with the multiplier in constraints also the horizontal center can be easily done with the same, so you could look for a good tutorial of Auto Layout and constraints for better understanding.
For iOS 9, an even simpler Auto Layout approach would be to use [UIStackView](https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIStackView_Class_Reference/).

As you can see, no constraints are needed for the buttons embedded in the stack view, as the stack view lays out the buttons for you. All you have to constrain is the location of the stack view itself.

Here's an example of two vertically stacked buttons, 10% from the bottom of the screen, and centered horizontally for all devices.

[![enter image description here](https://i.stack.imgur.com/izHwx.png)](https://i.stack.imgur.com/izHwx.png)