"Useless like tits on a log" vs. "tits on a bull" vs. "tits on a turtle" and so forth
Solution 1:
None of these variants are significantly more recognizable than the others. The importance of the phrase is "useless like tits on a..." and it doesn't really matter what the end of the sentence is as long as:
- The object has no tits
- OR the object does have tits but does not produce milk from those tits
That being said, "useless like tits on a nun" provides an interesting exception and is actually a clever play on the traditional phrase. Why this is the case is left as an exercise for the reader.