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 $100.78   8.221%   $109.0651   $109.07   $109.05   $109.10  
  Purchase #2 $18.96   6.481%   $20.1888   $20.19   $20.20   $20.20  
  Purchase #3 $168.42   8.109%   $182.0772   $182.08   $182.10   $182.10  
  Purchase #4 $46.51   11.375%   $51.8005   $51.80   $51.80   $51.80  
  Purchase #5 $221.56   4.375%   $231.2533   $231.25   $231.25   $231.30  
  Purchase #6 $122.73   7.866%   $132.3839   $132.38   $132.40   $132.40  
  Purchase #7 $190.49   11.785%   $212.9392   $212.94   $212.95   $212.90  
  7 purchase totals = $869.45       $939.7080   $939.71   $939.75   $939.80