Would my phone have to be reactivated after factory reset? I want to know if I  will have to reactivate my phone since it is already currently on through boost mobile. It is a LG tribute for which I paid a full $35 for a prepaid plan on June 27,2015. It is now July 3, 2015.
In your question, reactive might have two meanings:


Reactivation/registration is a process which is one time, at the time you purchased a device inserted your carrier SIM to make handset useable. It sends a message to the Device Company that this device is now active and registered.( This process is one time, your carrier may charge this message as per your plan.)
If you mean by setting up your device as new handset. So answer for this is


'Yes', you have to setup/register/reactivate all the things/accounts by yourself. Factory Reset will make your device completely clean(internally) as it was at the time when took out of the box.