Q: How do I know if I should wear a button down shirt tucked or untucked?
A: Length and fit will determine that. Below is an example of the right length for an untucked shirt. What you want is for the shirt to hit you no more than an inch below your hip crease. Proportionally, up to an inch and a half higher than the photo is good too but no lower or your legs will appear shorter than they are.
Also notice that the shirt’s fit is lean, also known as tailored, sport or European cut. Traditional or American cut shirts that fit looser around the torso should really be tucked in. 
You can submit your men’s style question to AskEmmi on Twitter.

Please do this if at all possible: http://thesartorialist.blogspot.com/2009/10/on-streetat-hemline-milan.html
This is key. Thanks for posting this. What is the “hip crease?” Also, are you talking about the very bottom of the back of the shirt, because usually the back of the shirt is the longest part.