In MLA style, should any part of a two-word preposition, such as according to, be capitalized in a title?

A preposition that is not at the start or end of a title should be lowercased, no matter how many words compose it and no matter how long those words are. A few examples:

according to
as regards
except for
other than

Some other styles capitalize a preposition or a word that belongs to a preposition if it has five letters or more.

If John Irving’s novel appeared in an essay or works-cited list published by the MLA, it would be styled The World according to Garp.