When writing papers on maths, when is it appropriate to use \rightarrow or \leftrightarrow, and when should one refrain from using them. For example, would this 
be appropriate use of \leftrightarrow.
EDIT:
What about this
Credit to Servaes, Peter Woolfitt and Michael, not trying to steal or anything, just using as an example. Should I make some arrows, or should I just line them up like in the picture.