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 $52.95   12.734%   $59.6927   $59.69   $59.70   $59.70  
  Purchase #2 $225.55   8.781%   $245.3555   $245.36   $245.35   $245.40  
  Purchase #3 $33.99   13.189%   $38.4729   $38.47   $38.45   $38.50  
  Purchase #4 $124.28   8.327%   $134.6288   $134.63   $134.65   $134.60  
  Purchase #5 $245.63   13.412%   $278.5739   $278.57   $278.55   $278.60  
  Purchase #6 $176.67   3.526%   $182.8994   $182.90   $182.90   $182.90  
  Purchase #7 $35.70   10.716%   $39.5256   $39.53   $39.55   $39.50  
  7 purchase totals = $894.77       $979.1488   $979.15   $979.15   $979.20