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 $53.80   5.622%   $56.8246   $56.82   $56.80   $56.80  
  Purchase #2 $11.41   9.186%   $12.4581   $12.46   $12.45   $12.50  
  Purchase #3 $98.52   12.469%   $110.8045   $110.80   $110.80   $110.80  
  Purchase #4 $59.39   3.767%   $61.6272   $61.63   $61.65   $61.60  
  Purchase #5 $22.49   3.531%   $23.2841   $23.28   $23.30   $23.30  
  Purchase #6 $212.44   3.330%   $219.5143   $219.51   $219.50   $219.50  
  Purchase #7 $99.54   10.662%   $110.153   $110.15   $110.15   $110.20  
  7 purchase totals = $557.59       $594.6658   $594.65   $594.65   $594.70