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 $25.95   3.560%   $26.8738   $26.87   $26.85   $26.90  
  Purchase #2 $16.86   4.546%   $17.6265   $17.63   $17.65   $17.60  
  Purchase #3 $230.80   8.288%   $249.9287   $249.93   $249.95   $249.90  
  Purchase #4 $127.66   8.580%   $138.6132   $138.61   $138.60   $138.60  
  Purchase #5 $110.49   10.288%   $121.8572   $121.86   $121.85   $121.90  
  Purchase #6 $25.70   10.753%   $28.4635   $28.46   $28.45   $28.50  
  Purchase #7 $51.35   11.846%   $57.4329   $57.43   $57.45   $57.40  
  7 purchase totals = $588.81       $640.7958   $640.79   $640.80   $640.80