Whenever you are in the process of repairing your credit you will in all likelihood be required to send some form of correspondence to creditors or the credit bureaus. Never send any form of correspondence without certified mail. If you have filed a dispute with the credit bureaus and have sent in the dispute while postal mail them sending it through certified mail will mean that you have proof of the correspondence in case there is no outcome to the dispute and no action is taken. Similarly if you have written a letter to the creditor to correct certain information that he is reporting to the credit bureau in his records than you will be in a position to prove such a request to the credit bureaus if he does not take the necessary action. The same holds true when negotiating with a collection agency or a creditor for the payment of an old due account.
Any form of correspondence that will be required later by you to prove your case to either the credit bureaus or in a court of law should be sent by certified mail to preserve it as a form of proof.