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 $128.71   11.385%   $143.3636   $143.36   $143.35   $143.40  
  Purchase #2 $221.48   7.492%   $238.0733   $238.07   $238.05   $238.10  
  Purchase #3 $38.83   12.371%   $43.6337   $43.63   $43.65   $43.60  
  Purchase #4 $111.91   7.331%   $120.1141   $120.11   $120.10   $120.10  
  Purchase #5 $153.34   11.188%   $170.4957   $170.50   $170.50   $170.50  
  Purchase #6 $45.35   12.693%   $51.1063   $51.11   $51.10   $51.10  
  Purchase #7 $11.75   12.475%   $13.2158   $13.22   $13.20   $13.20  
  7 purchase totals = $711.37       $780.0025   $780.00   $779.95   $780.00