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 $120.28   7.438%   $129.2264   $129.23   $129.25   $129.20  
  Purchase #2 $30.28   4.647%   $31.6871   $31.69   $31.70   $31.70  
  Purchase #3 $240.50   10.678%   $266.1806   $266.18   $266.20   $266.20  
  Purchase #4 $106.75   9.659%   $117.061   $117.06   $117.05   $117.10  
  Purchase #5 $82.89   8.222%   $89.7052   $89.71   $89.70   $89.70  
  Purchase #6 $31.82   12.446%   $35.7803   $35.78   $35.80   $35.80  
  Purchase #7 $132.28   5.733%   $139.8636   $139.86   $139.85   $139.90  
  7 purchase totals = $744.80       $809.5042   $809.51   $809.55   $809.60