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 $39.71   3.108%   $40.9442   $40.94   $40.95   $40.90  
  Purchase #2 $190.26   5.520%   $200.7624   $200.76   $200.75   $200.80  
  Purchase #3 $141.83   9.515%   $155.3251   $155.33   $155.35   $155.30  
  Purchase #4 $81.35   13.236%   $92.1175   $92.12   $92.10   $92.10  
  Purchase #5 $12.86   3.598%   $13.3227   $13.32   $13.30   $13.30  
  Purchase #6 $111.67   5.455%   $117.7616   $117.76   $117.75   $117.80  
  Purchase #7 $12.55   9.509%   $13.7434   $13.74   $13.75   $13.70  
  7 purchase totals = $590.23       $633.9769   $633.97   $633.95   $633.90