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 $227.97   9.642%   $249.9509   $249.95   $249.95   $250.00  
  Purchase #2 $61.73   4.568%   $64.5498   $64.55   $64.55   $64.50  
  Purchase #3 $17.96   4.474%   $18.7635   $18.76   $18.75   $18.80  
  Purchase #4 $62.81   3.282%   $64.8714   $64.87   $64.85   $64.90  
  Purchase #5 $182.90   10.119%   $201.4077   $201.41   $201.40   $201.40  
  Purchase #6 $175.58   13.630%   $199.5116   $199.51   $199.50   $199.50  
  Purchase #7 $30.58   3.543%   $31.6634   $31.66   $31.65   $31.70  
  7 purchase totals = $759.53       $830.7183   $830.71   $830.65   $830.80