The pronunciation of "を" in Japanese is "o"
In modern Japanese, the pronunciation of "を" is "o".
I feel sad even many Japanese misunderstand this point.
It seems that there are diarects where the "を" is pronounced "wo", but in the standard Japanese "を" is pronounced "o".
Please be careful when creating the UTAU sound source.
Is it really true?
You can be sure by listening carefully the lines of anime character or the speech of Japanese announcers.
Let me show you some sources just in case.
歌の場合、「を」を [β̞o̜] や [β̞o̜] で発音する歌手がしばしば見られる。しかし、語中・語尾の「お」も同様に発音され（中略）る例もあることから、母音連続を防ぐための用法とも見なすことができる。
The modern pronunciation: The same as お
In songs, we see often the singer who pronounce "を" as "wo". However, there are also the cases that "お" at the first part of a word or in a word is pronounced "wo". (Omission) Therefore it also is regarded as the usage to prevent continuous vowels.
At least in the modern common Japanese, there is no distinction between the pronunciation of "お" and "を"
In Roman alphabet, both "o" and "wo" are used.
"Wo" is generally used on the input, while "o" is used in the Hepburn romanization system.
Tips on creating UTAU library
Basically "お" and "を" have the same pronounciation, so I think the same phoneme is enough.
However, in general, we Japanese does not pronounce the postpositional particles strongly. It would be kind to make a dedicated weak phoneme.
Even if you use the same phoneme, an alias "を" helps UTAU users.