Is it safe keeping foreskin permanently retracted to avoid balanitis?

It is not a good idea to do this ( if you could actually succeed in holding the foreskin back permanently. the foreskin is there for a reason. If your foreskin is so tight that it will stay back from the the glans when pulled into that position it may well lead to restricting the flow of blood to the Glans which would have serious health consequences.

The best way to avoid the conditions that may be responsible for balinitis is to see to it that the foreskin is loose, ie able to be retracted and returned to its proper position easily and then insuring that it is kept dry and clean. exposing the glans of the penis permanently will lead to reducing the sensitivity of the glans, which in turn can lead to many sexual problems including premature ejaculation. and it may also eventually deny you the pleasure that is part of the nervous tissue inside the foreskin.