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 $52.91   9.591%   $57.9846   $57.98   $58.00   $58.00  
  Purchase #2 $120.63   13.110%   $136.4446   $136.44   $136.45   $136.40  
  Purchase #3 $80.36   10.637%   $88.9079   $88.91   $88.90   $88.90  
  Purchase #4 $108.58   13.776%   $123.538   $123.54   $123.55   $123.50  
  Purchase #5 $74.86   12.208%   $83.9989   $84.00   $84.00   $84.00  
  Purchase #6 $215.90   5.338%   $227.4247   $227.42   $227.40   $227.40  
  Purchase #7 $161.32   12.868%   $182.0787   $182.08   $182.10   $182.10  
  7 purchase totals = $814.56       $900.3774   $900.37   $900.40   $900.30