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 $99.33   5.120%   $104.4157   $104.42   $104.40   $104.40  
  Purchase #2 $28.66   3.473%   $29.6554   $29.66   $29.65   $29.70  
  Purchase #3 $159.94   10.791%   $177.1991   $177.20   $177.20   $177.20  
  Purchase #4 $11.40   13.714%   $12.9634   $12.96   $12.95   $13.00  
  Purchase #5 $51.40   12.268%   $57.7058   $57.71   $57.70   $57.70  
  Purchase #6 $73.70   9.400%   $80.6278   $80.63   $80.65   $80.60  
  Purchase #7 $34.26   8.513%   $37.1766   $37.18   $37.20   $37.20  
  7 purchase totals = $458.69       $499.7438   $499.76   $499.75   $499.80