No, it is not necessary if you are going to concentrate the lentiviral particles 24-48hrs post-transfection after harvesting cell supernatant twice. However, if you are going to use crude lentivirus in your next step and transduce target cells, the remaining DNA/reagent complex might be toxic to your cells.
If you don’t change the media and use crude lentiviral cell supernatant, there may be a slight increase in GFP positivity that could translate into a 1 fold higher titer. Therefore, you would just need adjust the MOI to infect your target cells.