Two important quations.... why are you cleaning it?... what are you cleaning?...
If you pulled old aquarium decor out of storage a warm water rinse should be plenty... if you collected rocks out of a river the cleanliness of the river needs to be considered and the solution may be anything from a warm water rinse to boiling... If plastic plants were in a tank that was known to have an infectious disease then using a mild bleach solution would be wise...
I personally would not use a bleach solution on a porous item such as a rock or wood, but wouldn't hesitate on a smooth hard plastic item...
1 part bleach / 5 parts water is a pretty strong bleach solution. I would be more likely to use 1/10, although this is not something I would maticulously measure. I'd simply approximate.
Bleach is essentially concentrated chlorine, which is why there is a suggestion to "dechlor" after using a bleach solution. I feel rinsing with dechlor after a warm/hot water rinse (no bleach) would be a waste of dechlor.
Also note that using bleach may remove color or whiten some things, such as driftwood...