Depends on the fish, with a mix you probably want both. Case in point: I have two banggai cardinals that eat out of the water column, I feed with the flow off so while they gobble up the freshly added stuff, once it's settled, they stop eating, and then when the pumps come back on, it kicks up some of the frozen they missed and they go dashing after it again. I've also got two mandarins which simply can't 'hunt' for the frozen food fast enough when it's blowing away in the flow, so I feed with the pumps off so they can get a crack at it, then after 5-10 mins turn it back on. The orange spot goby can eat in the full flow, but he seems to be much better at methodically getting pellets or chunks of things with the flow off, and then he often ascends in the tank to have a look - a behavior I've never seen with the pumps on full.
They definitely can be scared of a change, but they will likely quickly get used to it. I've been turning off my flow at around 10pm to try to catch spawns recently, and after a few repetitions a bunch of fish that normally wouldn't stray from the rockwork will travel around in the twilight because of the low flow conditions, and some of the other fish that would normally be out will be higher in the water column or in different parts of the tank than if the pumps were running.
