LOE is always advised, if your profile isn't straight forward.Did they asked you to submit webform for some kind of Letter of Explanation? I'm going to apply in April and don't know if the LOE should be submitted before process or officer would ask me for it.
For example, if you did two programs, 1+1years or, if you were part time or if you took some courses in scheduled break, stuff like that, you need an LOE.