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 $29.32   6.788%   $31.3102   $31.31   $31.30   $31.30  
  Purchase #2 $134.27   4.342%   $140.1   $140.10   $140.10   $140.10  
  Purchase #3 $173.74   7.837%   $187.356   $187.36   $187.35   $187.40  
  Purchase #4 $21.55   10.593%   $23.8328   $23.83   $23.85   $23.80  
  Purchase #5 $5.34   6.583%   $5.6915   $5.69   $5.70   $5.70  
  Purchase #6 $10.74   9.338%   $11.7429   $11.74   $11.75   $11.70  
  Purchase #7 $136.99   7.170%   $146.8122   $146.81   $146.80   $146.80  
  7 purchase totals = $511.95       $546.8456   $546.84   $546.85   $546.80