As above, relaly. Bactiria need ammonia to survie, and in the type of cycle that your fish shops are advising you to do, fish provide the ammonia. Ammonia is toxic, and at best will shorten the lives of your fish. A fishless cycle will be kinder on your fish, as an artificial source of ammonia is used, thus fish are at no risk of being poisoned. The second link in my previous post goes to the pinned topic on fishless cycling, and is well worth the read. It should make the process simpler to understang. Fish feed can be used instead of ammonia, if you cannot get the correct stuff, but bottled ammonia IMO gives more control over the cycle. As explained in the linked thread, if ammonia gets too high, bactiria production slows past 8ppm, and it can easily go beond this using food, thus stalling the cycle. If using food, you need to ba adding the ammount of food you expect to feed you fish dayly, for the fish you which to add at the end of the cycle.
HTH
Rabbut