Marines (with very few exceptions) all do IFS while in the Quantico area.
You can do it either before or after; my advice is to get orders to do it before TBS. I had my MOI get me orders to Echo Co (the rest of my graduating class went to Delta), but I reported to Quantico ASAP after graduation, checked into Mike Co, and started IFS. Wouldn't you know it - IFS ended for me the day Echo Co picked up, so no working parties for me...funny how that worked out

They also gave me "priority" on flights, since I was on a timetable to start TBS. I then completed TBS and got on the road to Pepsicola right afterwards. It is good, because a good portion of the folks I am going through API with were TBS classmates. Plus, I live an hour away (Milton) from P'Cola. With six months of waking up at 0400 or so to make the 0500 on-deck times, even with the hour commute, the 0700 start of API seems like sleeping in :icon_smil (In other words, I haven't had time to get used to sleeping past, oh, 0500...)
Those who did IFS after TBS had to stay attached to Mike Co (live at Camp Barrett if they are single :icon_rage ) and hang around the area they have grown to hate so very, very much. Plus, when they get down to P'Cola, they won't know as many people.