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 $159.93   6.721%   $170.6789   $170.68   $170.70   $170.70  
  Purchase #2 $172.76   3.121%   $178.1518   $178.15   $178.15   $178.20  
  Purchase #3 $57.83   8.824%   $62.9329   $62.93   $62.95   $62.90  
  Purchase #4 $27.50   4.804%   $28.8211   $28.82   $28.80   $28.80  
  Purchase #5 $99.88   8.574%   $108.4437   $108.44   $108.45   $108.40  
  Purchase #6 $25.73   9.459%   $28.1638   $28.16   $28.15   $28.20  
  Purchase #7 $60.25   5.774%   $63.7288   $63.73   $63.75   $63.70  
  7 purchase totals = $603.88       $640.9210   $640.91   $640.95   $640.90