For juvies and adult discus its usually not necessary to soak something like freeze dried blackworms as they soak up water fast. Krill and mysis should be ...its harder.
With small fish and fry its not a bad idea and something I do.One food I have always soaked is non-hatching decapsulated shrimp cysts...these can easily double in size as they soak up water. I have lost angel fry to them so always soak them about 10 -20minutes.
Hth,
Al