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 $21.42   5.455%   $22.5885   $22.59   $22.60   $22.60  
  Purchase #2 $78.26   13.700%   $88.9816   $88.98   $89.00   $89.00  
  Purchase #3 $137.51   9.599%   $150.7096   $150.71   $150.70   $150.70  
  Purchase #4 $101.89   6.468%   $108.4802   $108.48   $108.50   $108.50  
  Purchase #5 $44.49   9.649%   $48.7828   $48.78   $48.80   $48.80  
  Purchase #6 $207.76   3.387%   $214.7968   $214.80   $214.80   $214.80  
  Purchase #7 $195.57   6.589%   $208.4561   $208.46   $208.45   $208.50  
  7 purchase totals = $786.90       $842.7956   $842.80   $842.85   $842.90