"Playful" Sass

LoveEeyore

DD#1 is 5, just finished preschool.  She is a bright, kind, sweet girl.  She is polite and helpful.  We are so proud and in awe of her.  But.  Last few weeks she's been pulling a bit of sassy out and that's fine, she recants quickly with "the look."  But what is driving me crazy is when she gives a sassy "no" or "not fair" in a silly voice and laughs after. 

It is not a big deal in the grand, or even minor scheme, but I do get far more irritated at it.  I am ignoring at this point.  Would you ignore or treat it as the real thing?