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 $101.67   13.787%   $115.6872   $115.69   $115.70   $115.70  
  Purchase #2 $123.46   4.240%   $128.6947   $128.69   $128.70   $128.70  
  Purchase #3 $231.62   13.472%   $262.8238   $262.82   $262.80   $262.80  
  Purchase #4 $29.97   8.391%   $32.4848   $32.48   $32.50   $32.50  
  Purchase #5 $75.50   10.153%   $83.1655   $83.17   $83.15   $83.20  
  Purchase #6 $207.76   12.570%   $233.8754   $233.88   $233.90   $233.90  
  Purchase #7 $63.30   6.171%   $67.2062   $67.21   $67.20   $67.20  
  7 purchase totals = $833.28       $923.9376   $923.94   $923.95   $924.00