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 $27.34   10.730%   $30.2736   $30.27   $30.25   $30.30  
  Purchase #2 $225.80   12.351%   $253.6886   $253.69   $253.70   $253.70  
  Purchase #3 $104.57   8.274%   $113.2221   $113.22   $113.20   $113.20  
  Purchase #4 $55.33   4.225%   $57.6677   $57.67   $57.65   $57.70  
  Purchase #5 $247.83   6.128%   $263.017   $263.02   $263.00   $263.00  
  Purchase #6 $59.26   5.472%   $62.5027   $62.50   $62.50   $62.50  
  Purchase #7 $108.97   3.336%   $112.6052   $112.61   $112.60   $112.60  
  7 purchase totals = $829.10       $892.9769   $892.98   $892.90   $893.00