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 $28.40   8.688%   $30.8674   $30.87   $30.85   $30.90  
  Purchase #2 $79.56   5.811%   $84.1832   $84.18   $84.20   $84.20  
  Purchase #3 $47.83   5.177%   $50.3062   $50.31   $50.30   $50.30  
  Purchase #4 $42.29   5.444%   $44.5923   $44.59   $44.60   $44.60  
  Purchase #5 $92.56   11.456%   $103.1637   $103.16   $103.15   $103.20  
  Purchase #6 $182.29   8.346%   $197.5039   $197.50   $197.50   $197.50  
  Purchase #7 $64.32   6.523%   $68.5156   $68.52   $68.50   $68.50  
  7 purchase totals = $537.25       $579.1323   $579.13   $579.10   $579.20