Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $19.39   4.502%   $20.2629   $20.26   $20.25   $20.30  
  Purchase #2 $13.91   8.234%   $15.0553   $15.06   $15.05   $15.10  
  Purchase #3 $74.73   5.481%   $78.826   $78.83   $78.85   $78.80  
  Purchase #4 $45.98   13.357%   $52.1215   $52.12   $52.10   $52.10  
  Purchase #5 $155.74   5.824%   $164.8103   $164.81   $164.80   $164.80  
  Purchase #6 $176.28   7.103%   $188.8012   $188.80   $188.80   $188.80  
  Purchase #7 $176.88   6.263%   $187.958   $187.96   $187.95   $188.00  
  7 purchase totals = $662.91       $707.8352   $707.84   $707.80   $707.90