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 $10.52   3.106%   $10.8468   $10.85   $10.85   $10.80  
  Purchase #2 $36.83   8.488%   $39.9561   $39.96   $39.95   $40.00  
  Purchase #3 $45.84   11.173%   $50.9617   $50.96   $50.95   $51.00  
  Purchase #4 $127.83   9.508%   $139.9841   $139.98   $140.00   $140.00  
  Purchase #5 $82.88   7.365%   $88.9841   $88.98   $89.00   $89.00  
  Purchase #6 $78.85   10.723%   $87.3051   $87.31   $87.30   $87.30  
  Purchase #7 $109.84   11.638%   $122.6232   $122.62   $122.60   $122.60  
  7 purchase totals = $492.59       $540.6611   $540.66   $540.65   $540.70