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 $27.28   3.109%   $28.1281   $28.13   $28.15   $28.10  
  Purchase #2 $12.59   4.462%   $13.1518   $13.15   $13.15   $13.20  
  Purchase #3 $55.63   10.260%   $61.3376   $61.34   $61.35   $61.30  
  Purchase #4 $179.94   10.633%   $199.073   $199.07   $199.05   $199.10  
  Purchase #5 $28.29   12.272%   $31.7617   $31.76   $31.75   $31.80  
  Purchase #6 $211.38   11.153%   $234.9552   $234.96   $234.95   $235.00  
  Purchase #7 $216.71   12.712%   $244.2582   $244.26   $244.25   $244.30  
  7 purchase totals = $731.82       $812.6656   $812.67   $812.65   $812.80