A general search will search "every word" of the message and will pick any spelling for the names includes in the message but won't be able to determine whether they are surnames are used as a word, first name, locality, etc.
The Advanced Search SURNAME BOX search will only find what is entered in the surname box (provided it is entered correctly without extraneous slashes and dashes and such).
You MAY enter variant spellings of the surnames included in your message although it is best to also mention the variants in the message too. I wouldn't worry about the CDs that truncate the spelling of names--that isn't done with the surname box on the boards--so show the names in full but DO include variant spellings that someone might also be looking for. For example, my MYERS ancestors at various times on varions records spelled the name MEYER, MEYERS, and finally MYERS. Listing all three spellings will help people find my posts in a search.
It isn't REQUIRED that you enter all surnames in the message in the surnames box too-- but it is helpful for those who search later on if you do. Pay SPECIAL attention to surnames that could be confusing in a search: James, Joseph, Ireland, England, German, List, Cousin, Road...well, you get the idea.