If you are prone to keloiding, piercings are strongly urged against. Tattoos can also lead to keloiding, although piercings and surgical modifications seem to be the most likely culprits. There's no real limit as to where keloids can form, as they can also grow on the tongue and other mucus membranes. The only way to truly prevent keloiding is to not get the tattoo or piercing in the first place. If you decide to chance it, just be aware that you may end up with excessive scarring and/or keloids.
Keloids are formations of scar tissue that grow because the body over-defends itself as a result of trauma or surgical incisions. In most cases, keloids have to be removed with the help of medical treatment or surgery - you can't just wash them away. People with African-American descent tend to be more prone to keloids, although people of all ethnicities can be susceptible. Keloids also tend to be genetic.