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 $85.78   12.684%   $96.6603   $96.66   $96.65   $96.70  
  Purchase #2 $13.39   6.607%   $14.2747   $14.27   $14.25   $14.30  
  Purchase #3 $97.81   5.852%   $103.5338   $103.53   $103.55   $103.50  
  Purchase #4 $167.44   8.644%   $181.9135   $181.91   $181.90   $181.90  
  Purchase #5 $232.63   12.480%   $261.6622   $261.66   $261.65   $261.70  
  Purchase #6 $26.54   12.575%   $29.8774   $29.88   $29.90   $29.90  
  Purchase #7 $196.71   12.304%   $220.9132   $220.91   $220.90   $220.90  
  7 purchase totals = $820.30       $908.8351   $908.82   $908.80   $908.90