Maybe Jonathan's solution using multiple pills on the Text/Label Shelf is a viable way to workaround bug #258703 which you seemingly are facing:
In my case, the strings were ultimately only about doing some complicated formatting and I was able to avoid this entirely by taking advantage of 8.0's new ability to have multiple pills on the Text/Label Shelf, and then have individual custom number formats for each pill. Re: Error when converting rounded number to string
Ps. There are many threads about this rounding error: http://community.tableau.com/search?q=round+str
ROUND function in Tableau return different values from Excel rounding:
This type of rounding is called the Banker's rounding (see article http://kb.tableau.com/articles/knowledgebase/bankers-rounding). Also an extended explanation (and solution) is in this page Unexpected Results: Rounding | Drawing with Numbers.
Attached is my dashboard with an example of solution using a formula
or simply using the Tableau formatting that rounds correctly:
FixingRounding.twbx 66.3 KB