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 $119.46   13.532%   $135.6253   $135.63   $135.65   $135.60  
  Purchase #2 $53.61   12.581%   $60.3547   $60.35   $60.35   $60.40  
  Purchase #3 $17.78   12.870%   $20.0683   $20.07   $20.05   $20.10  
  Purchase #4 $236.82   11.763%   $264.6771   $264.68   $264.70   $264.70  
  Purchase #5 $29.51   8.668%   $32.0679   $32.07   $32.05   $32.10  
  Purchase #6 $199.41   7.185%   $213.7376   $213.74   $213.75   $213.70  
  Purchase #7 $55.40   8.358%   $60.0303   $60.03   $60.05   $60.00  
  7 purchase totals = $711.99       $786.5612   $786.57   $786.60   $786.60