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 $26.91   9.400%   $29.4395   $29.44   $29.45   $29.40  
  Purchase #2 $152.92   7.333%   $164.1336   $164.13   $164.15   $164.10  
  Purchase #3 $121.59   4.153%   $126.6396   $126.64   $126.65   $126.60  
  Purchase #4 $27.90   3.148%   $28.7783   $28.78   $28.80   $28.80  
  Purchase #5 $78.49   3.777%   $81.4546   $81.45   $81.45   $81.50  
  Purchase #6 $66.32   5.173%   $69.7507   $69.75   $69.75   $69.80  
  Purchase #7 $130.85   6.862%   $139.8289   $139.83   $139.85   $139.80  
  7 purchase totals = $604.98       $640.0252   $640.02   $640.10   $640.00