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 $18.40   9.502%   $20.1484   $20.15   $20.15   $20.10  
  Purchase #2 $40.86   12.294%   $45.8833   $45.88   $45.90   $45.90  
  Purchase #3 $210.93   6.142%   $223.8853   $223.89   $223.90   $223.90  
  Purchase #4 $11.76   11.325%   $13.0918   $13.09   $13.10   $13.10  
  Purchase #5 $105.99   8.514%   $115.014   $115.01   $115.00   $115.00  
  Purchase #6 $243.91   9.314%   $266.6278   $266.63   $266.65   $266.60  
  Purchase #7 $230.43   10.554%   $254.7496   $254.75   $254.75   $254.70  
  7 purchase totals = $862.28       $939.4002   $939.40   $939.45   $939.30