Cleaned coins are not really worth much if you try to sell them. If you are lucky enough to find a key date coin -16D Merc for example. You should not touch it as far as cleaning. It needs to be sent to a professional coin service for examination and recommendations. If you try anything the value drops like a rock. For everyday coins, I think people are saying olive oil is safe. It may take years for a coin to soak clean in olive oil. It just won't hurt the coin to stay in it.
Any rubbing of a coin will scratch it. Removing dirt with a camel hair brush, rinsing in soapy water and patting dry is about as safe as it gets for "regular" coins you just want to keep. You won't get them completely clean but they will be presentable. For clad coins, a tumbler and aquarium sand works good.
If the coins are corroded or have bronze disease. They are "dead meat" anyway so do what you like to try and clean them. Nothing usually works very well. Electrolysis for silver. Who knows what for the rest.
It really depends on how you want your coins to look. If you like the old patina, just wash with as little abrasion as possible and don't use any chemicals that will strip the patina. If you want them really clean and shiny, go for the chemicals, wire brushes, and buffing wheels. Most dug coins are worth much anyway so you aren't doing much as far as taking away value.
Nuf said?
Daryl