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 $110.73   9.720%   $121.493   $121.49   $121.50   $121.50  
  Purchase #2 $123.74   6.156%   $131.3574   $131.36   $131.35   $131.40  
  Purchase #3 $152.27   3.859%   $158.1461   $158.15   $158.15   $158.10  
  Purchase #4 $16.40   13.861%   $18.6732   $18.67   $18.65   $18.70  
  Purchase #5 $174.57   3.518%   $180.7114   $180.71   $180.70   $180.70  
  Purchase #6 $69.61   4.325%   $72.6206   $72.62   $72.60   $72.60  
  Purchase #7 $101.62   5.228%   $106.9327   $106.93   $106.95   $106.90  
  7 purchase totals = $748.94       $789.9344   $789.93   $789.90   $789.90