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 $230.54   13.453%   $261.5545   $261.55   $261.55   $261.60  
  Purchase #2 $207.31   7.421%   $222.6945   $222.69   $222.70   $222.70  
  Purchase #3 $123.52   12.445%   $138.8921   $138.89   $138.90   $138.90  
  Purchase #4 $123.55   12.533%   $139.0345   $139.03   $139.05   $139.00  
  Purchase #5 $216.87   12.269%   $243.4778   $243.48   $243.50   $243.50  
  Purchase #6 $229.41   12.857%   $258.9052   $258.91   $258.90   $258.90  
  Purchase #7 $62.54   11.259%   $69.5814   $69.58   $69.60   $69.60  
  7 purchase totals = $1,193.74       $1,334.1400   $1,334.13   $1,334.20   $1,334.20