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 $13.50   12.636%   $15.2059   $15.21   $15.20   $15.20  
  Purchase #2 $25.38   10.570%   $28.0627   $28.06   $28.05   $28.10  
  Purchase #3 $195.57   4.568%   $204.5036   $204.50   $204.50   $204.50  
  Purchase #4 $91.51   6.400%   $97.3666   $97.37   $97.35   $97.40  
  Purchase #5 $74.53   5.346%   $78.5144   $78.51   $78.50   $78.50  
  Purchase #6 $222.90   5.542%   $235.2531   $235.25   $235.25   $235.30  
  Purchase #7 $97.61   4.356%   $101.8619   $101.86   $101.85   $101.90  
  7 purchase totals = $721.00       $760.7682   $760.76   $760.70   $760.90