I learned in school that the hyphen is obligatory (but that was British English). When I googled it, I found the following grammar rule:

Hyphenate all spelled-out fractions.

Examples:
You need one-third of a cup of sugar for that recipe.
More than one-half of the student body voted for removing soda machines from campus.

Here is the link to the website:
http://www.grammarbook.com/punctuation/hyphens.asp
The author was an undergraduate at the University of California - it seems that this rule applies to the US, also.

I'm not sure that any one body rules on what is obligatory in AE, BE or any other version of English, least of all "an undergraduate at the University of California".

Having said that, I (BE) would use a hyphen.

Chicago Manual of Style, 16th edition, section 7.85:

fractions, simple: one-half, two-thirds, etc.

Hyphenated in noun, adjective, and adverb forms, except when second element is already hyphenated.