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 $247.61   10.452%   $273.4902   $273.49   $273.50   $273.50  
  Purchase #2 $39.76   5.183%   $41.8208   $41.82   $41.80   $41.80  
  Purchase #3 $61.45   4.218%   $64.042   $64.04   $64.05   $64.00  
  Purchase #4 $231.64   7.774%   $249.6477   $249.65   $249.65   $249.60  
  Purchase #5 $58.71   12.104%   $65.8163   $65.82   $65.80   $65.80  
  Purchase #6 $12.90   12.563%   $14.5206   $14.52   $14.50   $14.50  
  Purchase #7 $157.52   12.646%   $177.44   $177.44   $177.45   $177.40  
  7 purchase totals = $809.59       $886.7776   $886.78   $886.75   $886.60