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 $132.88   4.209%   $138.4729   $138.47   $138.45   $138.50  
  Purchase #2 $114.61   5.787%   $121.2425   $121.24   $121.25   $121.20  
  Purchase #3 $25.55   11.687%   $28.536   $28.54   $28.55   $28.50  
  Purchase #4 $34.54   9.697%   $37.8893   $37.89   $37.90   $37.90  
  Purchase #5 $79.76   8.489%   $86.5308   $86.53   $86.55   $86.50  
  Purchase #6 $28.63   12.744%   $32.2786   $32.28   $32.30   $32.30  
  Purchase #7 $8.79   10.686%   $9.7293   $9.73   $9.75   $9.70  
  7 purchase totals = $424.76       $454.6794   $454.68   $454.75   $454.60