In [7]: a = "ололо %s"

In [8]: a % u"бугага"
---------------------------------------------------------------------------
UnicodeDecodeError                        Traceback (most recent call last)
/home/meow-nofer/Projects/product/nailgun/manage.py in <module>()
----> 1 a % u"бугага"

UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128)

It's required if ascii meets utf-8. I would add it, in case of any problems, so we won't think about it in the future at all.