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 $19.91   6.523%   $21.2087   $21.21   $21.20   $21.20  
  Purchase #2 $213.48   10.519%   $235.936   $235.94   $235.95   $235.90  
  Purchase #3 $10.71   11.744%   $11.9678   $11.97   $11.95   $12.00  
  Purchase #4 $227.32   7.360%   $244.0508   $244.05   $244.05   $244.10  
  Purchase #5 $27.54   4.799%   $28.8616   $28.86   $28.85   $28.90  
  Purchase #6 $47.51   6.335%   $50.5198   $50.52   $50.50   $50.50  
  Purchase #7 $39.71   13.660%   $45.1344   $45.13   $45.15   $45.10  
  7 purchase totals = $586.18       $637.6791   $637.68   $637.65   $637.70