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 $20.62   6.614%   $21.9838   $21.98   $22.00   $22.00  
  Purchase #2 $71.51   7.199%   $76.658   $76.66   $76.65   $76.70  
  Purchase #3 $226.74   4.792%   $237.6054   $237.61   $237.60   $237.60  
  Purchase #4 $180.29   12.730%   $203.2409   $203.24   $203.25   $203.20  
  Purchase #5 $9.87   9.527%   $10.8103   $10.81   $10.80   $10.80  
  Purchase #6 $121.41   5.271%   $127.8095   $127.81   $127.80   $127.80  
  Purchase #7 $11.71   7.859%   $12.6303   $12.63   $12.65   $12.60  
  7 purchase totals = $642.15       $690.7382   $690.74   $690.75   $690.70