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 $46.54   12.388%   $52.3054   $52.31   $52.30   $52.30  
  Purchase #2 $49.66   13.176%   $56.2032   $56.20   $56.20   $56.20  
  Purchase #3 $74.87   4.696%   $78.3859   $78.39   $78.40   $78.40  
  Purchase #4 $93.28   11.728%   $104.2199   $104.22   $104.20   $104.20  
  Purchase #5 $180.37   9.500%   $197.5052   $197.51   $197.50   $197.50  
  Purchase #6 $247.73   5.794%   $262.0835   $262.08   $262.10   $262.10  
  Purchase #7 $83.92   7.132%   $89.9052   $89.91   $89.90   $89.90  
  7 purchase totals = $776.37       $840.6083   $840.62   $840.60   $840.60