When do I use V for Ü in Pinyin programs
In the past (e.g. 80's to 90's), when the Chinese IME did not implement fuzzy logic, the rule was you should use v for and only for ü as you would write on paper. For example for J you should type ju not jv.

Now all the mainstream Chinese IME (MSPY, Sogou, Google Pinyin, etc.) has fuzzy logic built-in and enabled by default. This question isn't much relevant anymore.