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 $28.84   7.685%   $31.0564   $31.06   $31.05   $31.10  
  Purchase #2 $13.80   11.773%   $15.4247   $15.42   $15.40   $15.40  
  Purchase #3 $68.34   9.557%   $74.8713   $74.87   $74.85   $74.90  
  Purchase #4 $125.62   11.507%   $140.0751   $140.08   $140.10   $140.10  
  Purchase #5 $228.43   9.389%   $249.8773   $249.88   $249.90   $249.90  
  Purchase #6 $163.35   7.691%   $175.9132   $175.91   $175.90   $175.90  
  Purchase #7 $199.63   12.733%   $225.0489   $225.05   $225.05   $225.00  
  7 purchase totals = $828.01       $912.2669   $912.27   $912.25   $912.30