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 $226.45   5.762%   $239.498   $239.50   $239.50   $239.50  
  Purchase #2 $75.86   9.506%   $83.0713   $83.07   $83.05   $83.10  
  Purchase #3 $69.84   4.588%   $73.0443   $73.04   $73.05   $73.00  
  Purchase #4 $209.69   8.421%   $227.348   $227.35   $227.35   $227.30  
  Purchase #5 $45.99   6.769%   $49.1031   $49.10   $49.10   $49.10  
  Purchase #6 $209.34   10.481%   $231.2809   $231.28   $231.30   $231.30  
  Purchase #7 $84.37   5.131%   $88.699   $88.70   $88.70   $88.70  
  7 purchase totals = $921.54       $992.0446   $992.04   $992.05   $992.00