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 $17.74   6.118%   $18.8253   $18.83   $18.85   $18.80  
  Purchase #2 $11.55   4.263%   $12.0424   $12.04   $12.05   $12.00  
  Purchase #3 $237.93   9.184%   $259.7815   $259.78   $259.80   $259.80  
  Purchase #4 $110.94   13.307%   $125.7028   $125.70   $125.70   $125.70  
  Purchase #5 $129.63   3.403%   $134.0413   $134.04   $134.05   $134.00  
  Purchase #6 $85.42   6.430%   $90.9125   $90.91   $90.90   $90.90  
  Purchase #7 $185.34   11.360%   $206.3946   $206.39   $206.40   $206.40  
  7 purchase totals = $778.55       $847.7004   $847.69   $847.75   $847.60