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 $9.79   10.175%   $10.7861   $10.79   $10.80   $10.80  
  Purchase #2 $65.38   6.327%   $69.5166   $69.52   $69.50   $69.50  
  Purchase #3 $19.84   11.645%   $22.1504   $22.15   $22.15   $22.20  
  Purchase #4 $242.87   7.382%   $260.7987   $260.80   $260.80   $260.80  
  Purchase #5 $223.79   12.268%   $251.2446   $251.24   $251.25   $251.20  
  Purchase #6 $46.40   4.560%   $48.5158   $48.52   $48.50   $48.50  
  Purchase #7 $217.89   4.572%   $227.8519   $227.85   $227.85   $227.90  
  7 purchase totals = $825.96       $890.8641   $890.87   $890.85   $890.90