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 $231.97   13.134%   $262.4369   $262.44   $262.45   $262.40  
  Purchase #2 $140.77   12.528%   $158.4057   $158.41   $158.40   $158.40  
  Purchase #3 $216.50   8.257%   $234.3764   $234.38   $234.40   $234.40  
  Purchase #4 $55.34   5.336%   $58.2929   $58.29   $58.30   $58.30  
  Purchase #5 $133.36   8.772%   $145.0583   $145.06   $145.05   $145.10  
  Purchase #6 $130.99   10.540%   $144.7963   $144.80   $144.80   $144.80  
  Purchase #7 $18.77   7.621%   $20.2005   $20.20   $20.20   $20.20  
  7 purchase totals = $927.70       $1,023.5670   $1,023.58   $1,023.60   $1,023.60