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 $243.76   12.386%   $273.9521   $273.95   $273.95   $274.00  
  Purchase #2 $214.44   11.185%   $238.4251   $238.43   $238.45   $238.40  
  Purchase #3 $141.45   9.255%   $154.5412   $154.54   $154.55   $154.50  
  Purchase #4 $182.84   5.559%   $193.0041   $193.00   $193.00   $193.00  
  Purchase #5 $159.48   9.462%   $174.57   $174.57   $174.55   $174.60  
  Purchase #6 $237.40   9.777%   $260.6106   $260.61   $260.60   $260.60  
  Purchase #7 $25.25   8.596%   $27.4205   $27.42   $27.40   $27.40  
  7 purchase totals = $1,204.62       $1,322.5236   $1,322.52   $1,322.50   $1,322.50