My friends in Kaohsiung just taught me the word for "taro" 芋頭.
I looked it up and found yu4 tou5 but my friends here insist it's yu4 tou2.
Is this a pronunciation difference between Taiwan vs the mainland?
This is actually one example of a more general phenomenon. Mandarin, as spoken in Taiwan, uses weak stress (the neutral tone) much less often than Mandarin as spoken in northern areas. This is true not only of Taiwan speakers, but of other areas as well. According to Jerry Norman's 1988 book, Chinese (p. 149):
Weak stress (neutral tone) is an essential feature of the standard language, and of most northern dialects, but because its occurrence is lexically determined it is difficult for non-Peking speakers to master. Speakers of putonghua from many regions in South China, however, employ only a very small number of weakly stressed syllables, and these are chiefly limited to grammatical suffixes and particles.
In my opinion, it is a regional pronunciation difference. I've checked the educational bureau dictionary of Taiwan, it is yùtou5.
However, it is often pronounced as yùtóu. And personally, I said this a lot too.
And most of the people just pronounce all as tóu here in Taiwan.
There'are two different pinyin of 頭 in Mandarin: |tou2| tou5|
頭|tou2|  [n] head; hair; hairstyle  [adj] first; top; start; end  [n] chief; leader; boss  [n] side; party; aspect  classifier for certain domestic animals 頭|tou5 suffix for certain objects, location words, and adjectives (to form a noun)| Example: 彩頭 cai3 tou2 頭髮 tou2 fa3 風頭 feng1 tou2 鐘頭 zhong1 tou2
All the example words contained 頭 has tone 2 reading. (I am wondering why they didn't include a tone 5 reading example.) It shows |tou2| is the primary pinyin of 頭