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 $111.32   3.644%   $115.3765   $115.38   $115.40   $115.40  
  Purchase #2 $51.62   5.434%   $54.425   $54.43   $54.45   $54.40  
  Purchase #3 $119.74   8.481%   $129.8951   $129.90   $129.90   $129.90  
  Purchase #4 $46.71   3.867%   $48.5163   $48.52   $48.50   $48.50  
  Purchase #5 $140.45   10.695%   $155.4711   $155.47   $155.45   $155.50  
  Purchase #6 $132.49   12.315%   $148.8061   $148.81   $148.80   $148.80  
  Purchase #7 $120.31   7.309%   $129.1035   $129.10   $129.10   $129.10  
  7 purchase totals = $722.64       $781.5936   $781.61   $781.60   $781.60