sounds like your LFS
wants to sell you more food
It is not necessary to feed even every day, much less twice a day. Most people here feed every other day because if you feed too often it will definately foul your tank (ie, high nitrates, phosphate, etc.) and then you'll have algae issues.
I have pretty high flow in my tank and my fish just like to play in the current. The only time I would be concerned is at night when they are resting, but If you have an area with caves, etc. they can still rest there.