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 $110.92   12.214%   $124.4678   $124.47   $124.45   $124.50  
  Purchase #2 $46.98   7.787%   $50.6383   $50.64   $50.65   $50.60  
  Purchase #3 $24.67   10.469%   $27.2527   $27.25   $27.25   $27.30  
  Purchase #4 $158.46   10.382%   $174.9113   $174.91   $174.90   $174.90  
  Purchase #5 $165.53   9.871%   $181.8695   $181.87   $181.85   $181.90  
  Purchase #6 $224.61   3.734%   $232.9969   $233.00   $233.00   $233.00  
  Purchase #7 $153.69   3.713%   $159.3965   $159.40   $159.40   $159.40  
  7 purchase totals = $884.86       $951.5330   $951.54   $951.50   $951.60