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4 Answers
4

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. 

v is not an Alphabet in official PINYIN system but an International Phonetic Alphabet.ǘ 读愚,于,雨,遇
But Sougou always using what is short for convenience. 
It's the most popular Chinese Input Method in China even Apple set it as default in OSX Mavericks. 
I think there are no strict rules,everything based on your own preferences.