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 $150.61   12.259%   $169.0733   $169.07   $169.05   $169.10  
  Purchase #2 $225.37   12.176%   $252.8111   $252.81   $252.80   $252.80  
  Purchase #3 $109.25   10.282%   $120.4831   $120.48   $120.50   $120.50  
  Purchase #4 $25.54   4.585%   $26.711   $26.71   $26.70   $26.70  
  Purchase #5 $168.45   4.121%   $175.3918   $175.39   $175.40   $175.40  
  Purchase #6 $125.68   3.386%   $129.9355   $129.94   $129.95   $129.90  
  Purchase #7 $122.42   13.212%   $138.5941   $138.59   $138.60   $138.60  
  7 purchase totals = $927.32       $1,012.9999   $1,012.99   $1,013.00   $1,013.00