There are some rocks that aren't safe to use...it's true. You shouldn't use rocks with veins of metal in them, rocks like fools gold. Certain rocks will up the pH, like limestone (which may be a good or bad thing depending on your fish). Certain rocks are sharp, like lava rock...delicate fish might scratch themselves. And then there is the weight issue - if you're going to be piling on a lot of rocks, you would be wise to use some kind of protection under your substrate, like egg crate (light diffuser panel sold at hardware stores) cut to fit the tank's dimensions, or even an undergravel filter plate (if you happen to have old ones laying around, the way I do!)
Let me ask you something....do you always believe what the "pet store lady" says?
If so, then you're going to end up with aquarium salt, disposable carbon cartridges, and decorations from the pet store. HEAVEN FORBID you pick up your own,
free, decorations from nature.
Driftwood can take some time to treat, but weather worn smooth rocks that are non porous, are going to need nothing but a rinse under running water. You can scrub any caked dirt with a toothbrush, and if you're feeling paranoid, you can boil them, but this is way overkill.
Most aquarists use rocks from the wild, driftwood too...although that does need some boiling and can absorb more nasties (fertilizers, pesticides) than non porous rocks. I know of one guy who killed all his fish after introducing some driftwood (an oak stump) from his yard into the tank...he didn't realize that the neighbor had been spraying his lawn for chinch bugs and he got a piece that was bordering his and said neighbor's property
