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 $36.33   7.790%   $39.1601   $39.16   $39.15   $39.20  
  Purchase #2 $28.36   6.598%   $30.2312   $30.23   $30.25   $30.20  
  Purchase #3 $50.61   10.460%   $55.9038   $55.90   $55.90   $55.90  
  Purchase #4 $236.86   7.336%   $254.236   $254.24   $254.25   $254.20  
  Purchase #5 $138.38   11.646%   $154.4957   $154.50   $154.50   $154.50  
  Purchase #6 $207.74   7.331%   $222.9694   $222.97   $222.95   $223.00  
  Purchase #7 $221.65   3.437%   $229.2681   $229.27   $229.25   $229.30  
  7 purchase totals = $919.93       $986.2643   $986.27   $986.25   $986.30