This really depends on what your intentions are. If you plan on filing a lawsuit in the future against possibly a CA reporting inaccurate info, but refuses to delete. Then you want to be able to show as much documentation that you have bent over backwards trying to get the information corrected. If you know that under no circumstances this will become necessary then regular mail will do.
I have heard that sending letter CMRRR to the CRA's can cause the CRA's to begin to believe you are working with a credit repair organization. You really don't want them to think this because once they do your repair could halt.
Maybe to be on the safe side you could send it certified mail w/ no return receipt requested. I always suggest handwriting your disputes on notebook paper. Many people suggest writing it on yellow legal pad with purple writing. The reason this is done is because the CRA's are not really reading these disputes. They are scanning them into a computer. So handwriting and using highlighter is thought to cause the CRA's to have to actually read the disputes.
I really don't suggest online disputes after your first initial dispute. The first dispute usually results in the most deletions because of the "low hanging fruit". When you go online and submit disputes you are keying in all the information for them. Especially, the not mine disputes. All that is happening is that they are doing an electronic verification of certain factors such as if; your name matches, address matches, dob, ss#. The CRA's are not really speaking to the OC's at all. So a "not mine" dispute usually will not go far after the first dispute. Sometimes you may get lucky and the OC or CA won't respond.
The first thing you need to be doing is having the CRA's to delete all former addresses and update your current address. Also, have them to delete any name variations which are not accurate. Once this is complete start your disputes. This way at least if the trade line you are disputing is tied in to one of those incorrect addresses or names then it has more of a chance to be removed. If this mis-information is still on your report and a trade line is connected to it then its likely to get verified electronically.
I hope this helps. Sorry to go off on a rampage.
IN GOD WE TRUST ~ ALL OTHERS MUST DOCUMENT!