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 $25.99   10.505%   $28.7202   $28.72   $28.70   $28.70  
  Purchase #2 $171.75   4.441%   $179.3774   $179.38   $179.40   $179.40  
  Purchase #3 $214.49   8.841%   $233.4531   $233.45   $233.45   $233.50  
  Purchase #4 $65.79   5.268%   $69.2558   $69.26   $69.25   $69.30  
  Purchase #5 $92.66   8.519%   $100.5537   $100.55   $100.55   $100.60  
  Purchase #6 $21.71   7.735%   $23.3893   $23.39   $23.40   $23.40  
  Purchase #7 $11.73   10.122%   $12.9173   $12.92   $12.90   $12.90  
  7 purchase totals = $604.12       $647.6668   $647.67   $647.65   $647.80