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 $247.72   7.488%   $266.2693   $266.27   $266.25   $266.30  
  Purchase #2 $108.92   8.121%   $117.7654   $117.77   $117.75   $117.80  
  Purchase #3 $153.61   9.398%   $168.0463   $168.05   $168.05   $168.00  
  Purchase #4 $30.82   6.423%   $32.7996   $32.80   $32.80   $32.80  
  Purchase #5 $192.72   4.556%   $201.5003   $201.50   $201.50   $201.50  
  Purchase #6 $15.68   8.119%   $16.9531   $16.95   $16.95   $17.00  
  Purchase #7 $172.35   12.509%   $193.9093   $193.91   $193.90   $193.90  
  7 purchase totals = $921.82       $997.2433   $997.25   $997.20   $997.30