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 $249.39   9.456%   $272.9723   $272.97   $272.95   $273.00  
  Purchase #2 $46.43   11.147%   $51.6056   $51.61   $51.60   $51.60  
  Purchase #3 $33.68   3.484%   $34.8534   $34.85   $34.85   $34.90  
  Purchase #4 $161.63   5.242%   $170.1026   $170.10   $170.10   $170.10  
  Purchase #5 $76.30   9.450%   $83.5104   $83.51   $83.50   $83.50  
  Purchase #6 $169.35   9.486%   $185.4145   $185.41   $185.40   $185.40  
  Purchase #7 $209.89   9.751%   $230.3564   $230.36   $230.35   $230.40  
  7 purchase totals = $946.67       $1,028.8152   $1,028.81   $1,028.75   $1,028.90