I always have it prepared. I usually start the fight with charging FLUDD... FLUDD is good at throwing off opponents and decent at keeping them offstage. It works best when your opponent is mid-recovery, helpless, or stuck in a committed move (Falcon/Warlock Kick, Sonic/Sheik/ZSS d-air). It also works great with cape, and while not always easy to pull off, can lead to some dirty plays.
For example:
https://www.facebook.com/FrankRR3/videos/1104538136229070/?l=2732462717524224424
(please ignore the terrible quality and my derpy voice at the end)
With that being stated, FLUDD is also good against opponents who enjoy camping, or rushing in (Duck Hunt, Mac, C.Falc, Megaman, Pac-Man, etc.) because it tries to give you control of the stage. Also, if used correctly, it could even launch them a bit into the air and allow you to get some up-air juggles or even a forward air off-edge because they're not expecting it. Sure, shielding prevents FLUDD from doing anything, as does air dodge, but FLUDD literally takes 3 seconds or so to fully charge, and can even be useful at half charge. It can also be cancelled immediately into shield, so it's fairly safe to charge against defensive/campy characters and aggressive alike (as long as they aren't grab-happy, in which case, shield-cancel into sidestep or jab).
My big issue honestly is finding a way to get in on a defensive Rosalina, as her down+b absorbs FLUDD entirely, and that's one of Mario's best ways for getting in on and/or punishing campy players. Rob is also a nightmare to get in on, and a good Shulk can keep you out for days. The only difference is that the latter don't absorb saltwater like it's candy.
Edit: Small additions and grammar fixes.